I have an idea of what that reason might be...
As you said, the current state of the NBA makes it easier for guards to score. Defenders can't hand-check anymore. Incidental contact is frequently credited as a shooting foul for the offensive player. The looseness of the way the illegal screen rules are being interpreted boggles my mind. The Warriors run of wins last season was partially the result of exceptionally talented shooters in a well-designed offense and partially the result of hundreds of moving screens that went uncalled. There's a clear mandate now: the NBA brand is entertainment, always has been. People like what the Warriors are doing, the streak was good for the league as a whole, so they're going to continue to get away with it. An aggressive guard who can force the action off the dribble and make enough of their jumpers to be considered a threat at the three point line is very hard to defend. That same player coming off what I call a bump-screen, that is a screen followed by a subtle or often not-so-subtle hip bump into the defender, is impossible to defend.
Obviously James Harden, Steph Curry, and Russell Westbrook are very good players but collectively they're now reaching heights of statistical majesty we haven't seen in 30 years if ever. That's not a coincidence. The rules changes are an important factor too. Which shouldn't take anything away from their achievements! All players are judged on the standards of their era. I think that's what older players are trying to say when they grumble that today's stars wouldn't be all that in the league they played in. They know the rules so much better than we ever could and I guarantee you the differences are night and day to them. So how does this impact big guys? I don't think scoring in the post is disadvantaged now except that the relative value of a 2pt field goal becomes less significant as the rate of three point shooting increases. Whereas in the past you could ride a big guy who's automatic in the post, nowadays you'd be trading two for three on so many possessions that it's easy to fall behind. But the real problem is broader than that. It's a team building problem...
Why is it hard to land two stars on a team? Because of the way new talent is distributed into the league. The draft rewards teams with the worst records and the lottery throws a wrench of uncertainty into the works. If you get your one star, they're going to elevate you above the basement level teams who have no stars and there's always about 4 or 5 of those teams at a time. Picking at #5 and lower in the draft very rarely results in drafting a franchise talent. We got one with Boogie but it was a bit of a fluke. Once you have your star your baseline level moves up a notch from 15 win territory into 30 win territory. Hello mid-lotto purgatory! In the past the general rule was that you go big over small, all else being equal. Get your cornerstone big guy to score in the post (still the easiest place to score on the floor) and prevent other teams from scoring in the post on your end. This is the basic logic we've all been assuming to be true (well, those of us old enough to remember a time before the hand check rule changes). What we're seeing now though is that teams with one star guard are finding success while teams with one star big are not. All the rules changes implemented to speed up the game, open up the floor, and increase scoring have reversed the historical trend and the new rule should be that you always go guard/wing over forward/big, all else being equal. The players who create off the dribble and shoot from outside are the hardest to stop and the importance of locking down the paint on defense has taken a back seat to stopping guard penetration and three point shooting.
So here's the lynchpin -- it isn't actually any easier for guard-led teams to add a second star than it is for big-led teams. But it is easier for guard-led teams with mediocre supporting talent to make the playoffs right now than it is for a big-led team with mediocre talent to make the playoffs. Except in rare circumstances (draft luck, three amigos pacts, jackpot Free Agent signings) it's still rare to see a team with a franchise level guard and a franchise level big guy at the same time. With all the advantages going to guards who can actually benefit from the increased freedom to handle the ball in space, the allowance of illegal screens, and the benefit of the doubt often going to the quicker, more aggressive player (think of how many fouls James Harden gets just because the ref can't see what's going on and imagine him getting those same calls moving at half the speed) it all adds up to a guard dominated league and that means you're better off planting your flag on a franchise level guard and slowly building respectability from there. Golden State was able to lure Igoudala just by making the playoffs with a Curry/Thompson duo. Boston parlayed their success with a revolving door of attacking guards and wings into an Al Horford signing this off-season. Portland... well the needle scratched on that one. But there's also Cleveland who's been the best team in the East with Kevin Love and a smattering of role-players in their front court. Or Toronto with Lowry, DeRozan and ... quick name 2 other players on their team! Not to mention the lightning in a bottle Houston has stumbled upon with Harden playing the Steve Nash role in D'Antoni ball.
Or this could all just be rationalization. I will say, however, that the teams who have made out like bandits from the current rule changes have almost without exception done so completely by accident. Houston got Harden because he was ripe for the taking and they had the assets to make the trade. Taking Curry was a no-brainer for Golden State and they're lying if they say they knew at the time he was going to be an MVP. Westbrook is just who OKC ended up with after their cheapness let every other star leave town. And so on. Now this sounds like I'm making a case for "it's not our fault" which isn't really true. Mismanagement took what should be a sure-thing playoff team and shed essential pieces every step of the way until there's nothing left but Cousins. This picture says it all:
In a couple weeks Rudy's .07 spg lead will evaporate or he'll no longer meet the minimum game threshold and it'll be a clean sweep of Cuz faces leading the team in every statistical category. We've systematically expunged every potential supporting player we managed to acquire through bad luck or bad decision-making and we have no one but ourselves to blame for that. This is what failure looks like -- a dozen smaller failures any of which could be explained away at the time all adding up to the dark night of the soul where we accept once and for all that we caused all of this or we continue to pursue business as usual and see where that leads us.