Because they've grown up trusting the dollar. You have to understand that for most people in the world, dollars are the best store of value they've known because their own currency debases faster. But sadly, most don't have access to dollar bank accounts, and if they save at all it's with cash. That doesn't work very well because it's easily stolen or lost, so for them, USDT is an upgrade over cash. They don't know much about Tether the company or the trust they're placing in the many custodians, they just know its value stays the same in dollar terms.

In other words, the gap between what they already trust (dollars) and what they're asked to trust (USDT) is smaller than the gap between what they don't trust (BTC) and what they're asked to trust (BTC again). But the gap between USDT and BTC isn't nearly as big, which is the point of my comment.

If you want to see how stablecoins and BItcoin are perceived on the ground, go to a country like Turkey or Indonesia and talk to the people actually using it. Many of them get into Bitcoin after some good experiences with stablecoins.

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