From my experience I just think it's very difficult for a smaller player to take over a game to the extent that a larger player can. It's easy for a playoff defense to suffocate them and harder for them to make an impact on the defensive end. If you had to name the best players under 6'4" over the last 20 years, the ones who show up the most on All-NBA teams - Nash, CP3, Rose, Westbrook, Dame - what do you notice they all have in common? Not one of them ever made the Finals as a #1 option. The only guy I left out is the one who did it - AI - and he was a fukking unicorn and still only managed one Finals appearance.
I'm not saying shorter players are inferior or less skilled or anything. It's just that playoff basketball is that much more difficult for them. It's no accident that most people's all-time Top-10 lists don't include anyone under 6'5", even though 99% of folk who ever pick up a basketball are 6'5" or under.