Because the price of bitcoin is getting higher and higher.
Faucets earn from ads and ad companies usually pay with dollars not with bitcoin.
So the faucet owners have to adjust their rewards according to how much the market gives them.
It's not just ad revenue, many faucets are now offering gambling too...which is paid for in bitcoin.
It's a pretty clever model, especially presuming that any of these faucet owners believe in the long-term success of bitcoin. They can enforce a diminishing cost of doing business if their expenses are paid in terms of dollars while revenue is earned in bitcoin.
I remember when faucets were paying a 1000+ satoshi per hour easily! Back in my day....haha.