I think that it’s a mix between some of the following:
(1) They simply don’t like the way he plays.
This is a valid reason, as strange as it is, but it happens all the time. We all like certain things, certain positions, certain skills more than others, and therefore we tend to like some players more than others, regardless of how good they are.
I tend to like defensive midfielders and wingers less than I like “natural” No. 10s, for example.
(2) They somehow believe that Messi playing for the opposing team, or the longtime rivals, means that they can’t respect him. This is notoriously common among sports fans.
One of my favorite feelings in the world is when a player that causes trouble to the teams I support becomes so good that I don’t even care who he plays for. Cuauhtemoc Blanco, Zinedine Zidane, Cristiano Ronaldo, Wesley Sneijder (against Uruguay), and even Messi (against Mexico and Uruguay). Beckham was my favorite player and he remained in that spot when he went to play for Madrid.
(3) They don’t like his personality or his history, and can’t separate the that with his abilities as a footballer.
We like athletes and artists because of their set of skills in what they do, not because of their political opinions or the way they treat other people. It’s also not healthy to put “celebrities” on a pedestal.
They are just people like us, with qualities and shortcomings, with great skills in a certain area. Nothing else.
(4) They buy into all the bad publicity that certain media gives to Lionel.
(5) They think that Messi owes something to them, and has failed to fulfill those expectations.
Mostly people from Argentina, but I suppose there are people from other places who suffer from this.
(6) They don’t appreciate how good he is.
Not everyone knows enough about football to be able to distinguish between a good and a bad player, a great and a good one, an elite player and a superstar, and even between superstars.
Once upon I time, I didn’t get how people could like Sergio Busquets or Patrick Vieira. Right now, with almost 20 years of following and playing football, I still fail at it all the time.
We, as fans, wouldn’t be able to appreciate all of that if not for all the qualified journalists, writers, commentators, pundits that explain everything that’s happening.
There’s even a career that revolves around studying players at a very young age to determine/predict if they are/can be good enough to become useful for a professional club at some point. It’s called scouting, and there are people on Quora (follow Greg Gordon) who can explain better how difficult it is and how much you need to know to be able to make those predictions accurately.
But there’s no denying that Messi is one of the best players we have seen, whether people like it or not.
Instead of hating, I’d suggest some people tried to appreciate this magical player while we still can. It’s a blast.