Despite the frequent complaints levelled at the system on forums and the like, players seemed content with how martials operated at the time that this data was pulled from D&DBeyond. Perhaps WotC saw this and somehow felt Fighters were in a good place while Sorcerers and Druids needed some juice?
There's two critical flaws with this argument: First, It assumes that the
only reason people would commonly play a class is that they are enthusiastic about
every part of it. This ignores several known other effects, such as "new players should play Fighters because they're simple and thus won't overwhelm newbies," and "some people just don't like playing spellcasters, but almost all classes in 5e have access to spells." Second, it assumes that 5e players are
one monolithic unit, not (as we know them to be) several very distinct groups who may have very little in common
other than the fact that they play the same game.
It could be that Fighter is just
so gosh-darn popular as an archetype--regardless of its good or bad instantiation--that it simply rises to the top of its very nature. We can actually see a perfect example of this effect in 5e, one where WotC
did eventually "admit" that there was a problem and fix it:
dragonborn. People complained (I would know! I was one of them!) that 5e dragonborn were really, really weak--stupidly so,
much weaker than any other race in the PHB. Yet for years, people
assumed this was fully 100% intended, to the point that people trying to "reconstruct" the "rules" for how races are built explicitly told me that they basically had to assume that having elemental resistance was considered a HUGE bonus (like, the equivalent of an entire feat
by itself). Yet, despite this weakness...dragonborn were popular. Very popular. In fact, they only
grew in popularity. Early D&D Beyond statistics put dragonborn in 5th place, and over the next two or three years (not exactly sure) they to 3rd or 4th depending on exactly how you slice the data....and all of that was
before Fizban's gave us the updated dragonborn options. Dragonborn were popular
despite their weakness (which WotC has now officially addressed), not
because of that weakness.
Likewise, it could be that certain segments of the D&D market are really,
really happy with the Fighter (and thus play it a lot), and other segments are really,
really not happy (and thus complain about it a lot). We can see this, for example, with the response to some options: the Beast Master Ranger (almost surely THE most likely class AND subclass to get heavy reworks in 5.5e), or the vehemently negative response to the proposed Spell Versatility optional feature for Sorcerers
which came almost exclusively from people who don't play Sorcerers, or the notoriety of the honestly-not-THAT-bad Twilight Cleric.
And that doesn't even touch on the "make newbies play
the class that sucks most the simple class because we don't want to scare them away" sort of things, the extensive cultural baggage associated with certain classes (like Wizard and Fighter) that
will push people toward playing them even if they're actively bad. (Note, I'm not saying the 5e Fighter
is actively bad--just that even if it WERE so, it would STILL get played, possibly a lot, because of its cultural significance.)
Finally...it could be the case that lots of people really like Action Surge (because it can be quite potent as a combat feature) and certain other aspects, even if they're quite disappointed with
other aspects. This kinda connects with the dragonborn example above: people may be willing to put up with a lot of things they don't like simply to have "Fighter" written on their character sheet or to have Action Surge, Indomitable, and Second Wind. It might be that certain features are liked well enough that even vehement dislike of some other aspect is not sufficient to push people away.
In practice, it will almost certainly be a melange of all these effects. Some people are just diehard Fighter players and will play it even if it sucks. The market
is segmented, even if WotC would like to pretend that it's all one big happy family, and that can lead to situations where 20% love it, 20% hate it, and 60% just don't care enough to say anything. There are quite probably some features that people would like to see changed--
Mearls himself said that one of his biggest regrets was how little flavor the Fighter class had, for example--even though Action Surge and Indomitable are desirable features. There definitely are cultural factors pushing people toward the Fighter. Etc.