This is a huge one, and the thing I think is the biggest factor. For whatever reason, critics were on John's nuts and subscribed to the groupthink that Paul was a soft square, and they blamed him for the Beatles' breakup...it became the conventional wisdom and the consensus...it certainly was when I grew up (in the 1980s/90s). I broke free from that mindset, luckily, but I think as you said, so many people now that are younger don't have any of that baggage and so appreciate it for what it is right off the bat, without any preconceived notions.
One word why: Vietnam. That generation never got over that conflict, and they're still more or less fighting over it decades later in politics over other issues. Remember when John died, he was eulogized as the peace activist...even though he had quit that for 7-8 years before he died, in fact he was already speaking of dissillusionment with that era. Of course for a whole new generation of critics and fans, Vietnam for them just means nothing. It's just that place Sylvester Stallone in that 80s action movie went to and killed everybody. (Or that war which America lost several wars ago.) Then add two little, other elements: (1) Paul for a time was blamed for breaking up the Beatles because of his lawsuit, though now we know the whole story. (2) First post-Beatles solo albums by George and John were homeruns. I like MCCARTNEY, but...its not a homerun album. If Paul had taken the time to crank out some good songs instead of instrumental jams, instead of wanting to stick it to Klein and his ex-co-workers, maybe his solo career would've been better received as a whole?