>>51438378This is to make Eevee into a more marketable mascot to sell more product, much easier to do when you only have one Pokémon to shill in merch instead of 8. It also gives Eevee a central role as a mascot of the company, if everyone evolved it this wouldn't be the case for obvious reasons.
Next time you have a question like this, remember that GameFreak is a company, and a huge one at that, almost everything they do is for money purposes.
