Unlike Antpool, Foundry is *NOT* a 'public' pool that anyone can use. It is is for their private mining and for Institutional miners who contract with them. The attraction for Institutional miners is simple: Dirt cheap rates and no equipment to buy.

Now as to popularity of Antpoo - no idea. Yes they too have Institutional contracts and with their US operations can offer them rates on-par with Foundry and MARA and I'm certain that a large part of their size are Institutional customers. Question there is why many much smaller miners use Antpoo? Frankly, for small miners their payout scale suxs...