If you have a great and transcendent SG/SF or C, you can put a solid (but not spectacular) team around them and win a ring. Dirk, Duncan, Wade, Hakeem etc., are examples of players that took slightly above average teams that might win 40 games without them, and push them over the top.
A PG can be the best player, but they need a super-team around them for it to work. Curry would not carry the '94 Rockets, '06 Heat or '11 Mavericks to a title. However if you put him on a strong team that can accentuate his strengths and mitigate his weaknesses, he's great enough as a player to lead you to a ring.
SG/SF's and C's that lead teams to titles are usually prolific scorers that may or may not be good defenders. PG's are basically the QB's, so not only do they have to distribute the ball and set things up, they'd need to be great scorers that can work within the offense and decent defenders. Few PG's have that broad of a skill-set.
Of course the term "PG" is a bit misleading. LeBron has pretty much always operated like a PG, even though he's not in the traditional sense. I think what we all mean is a floor general that's 6'4" or shorter and generally "looks" the part of a PG (Chris Paul, Steve Nash, Gary Payton, Jason Kidd, John Stockton etc.).