The issue with time-limited skins: a lot of players change mains over time
I tend to bounce around on who I'm "maining" quite a bit when I play hero shooters: I'll enjoy a certain set of characters for a time, then that'll shift as my time with the game continues. Who gets played can also be heavily meta-dependent, as characters shift in and out of favor.
The only problem is that I'll often "fall in love" with a certain character later in the game's life, only to find that I can't buy any of their best skins when I otherwise would have.
It's just not financially feasible for the average player to buy every skin released in the off chance that they wind up really liking a character later on.
Maybe FOMO drives more profit (it probably does), but it just sucks to gain a newfound love of a character later on only for a huge number of their cosmetics to be unavailable for purchase....