I think you are really underestimating how big a deal magic guard is. Psychic is a p terrible type defensively, rendering mew's bulk almost unusable given all the crunches, uturns, and pursuits flying around. Mew is generally outclassed in the lead spot by faster, harder-hitting stuff like chomp, terra, and even azelf etc. So mew is forced to go offensive to remain viable. We've seen the same thing happen with reuniclus, whose CM set has dwindled while its OTR one remains much more popular. However, Alakazam is simply much better than mew in the offense department. While mew has a better movepool, alakazam has MUCH better speed and power, and coverage that, if not as finely tailorable as mew's, is perfectly sufficient for sweeping/attacking purposes. Psychic from alakazam just hits way way harder than from mew. And of course, alakazam has magic guard over mew, which either allows it to run a set that is far far more powerful than any of mew's without recoil, or to run the ultimate focus sash set, which is close to guaranteed a KO a game. Meanwhile, if mew wants to be any kind of offensive threat it needs to sacrifice bulk, meaning that its 2hko be p much everything...while alakazam is guaranteed a 2hko at worst while still hitting way harder. Sash alakazam is also hugely potent at high levels of play, as a mon that can revenge any threat no matter how many hazards are up or how many boosts they have can turn the tide of games. So mew is still decent but just doesnt fit well in this meta, while alakazam has a definite niche and advantage over mew.