>>56947526The mechanical reason is
>>56947674, the lore reason dovetails with that with the implication being that evolving past a certain point is either unnecessary for the monster (a lot of them don't even evolve at all) or the creatures would need to assume such hyperspecialised forms that they're not fully sustainable (the combat prowess-focused Mega Evos are only possible with outside help in the form of the power of a bond with a trusted trainer and energy from mega stones, don't last very long and in many cases cause the Pokemon in question some amount of distress).
I would not be surprised if in the future some special Pokemon family is designed whose entire gimmick is evolving three times without the final evo being temporary. Maybe another egg-themed Pokemon like the Togepi line, with the first in line barely having hatched and needing all those additional stages to reach full maturity.
