To understand the details of it you have to comb through the material spread across a dozen different mediums.
But the reason for it's popularity is simple.
You have Historical and Mythological figures thrown into (usually) a battle royale so you can argue about matchups and class changes for years.
The magic system is has a few holes but it's very thought out.
It's part of a larger universe with deepest lore for ages. Even Prisma goddamned Illya has lore and it's selling point is oversexed middle schoolers.
And of course waifus for every taste and most fetishes.