Tether is popular because it was the first stablecoin by several years (e.g. Tether launched in 2014, USDC launched in 2018). Much like Bitcoin, Tether isn't a particularly good product nowadays if you look at its fundamental properties - most of its usefulness comes from its extrinsic properties such as liquidity and demand. However, much like Bitcoin, due to its very early launch, Tether became an early schelling point that has gathered significant liquidity/usage due to network effect (liquidity begets liquidity).

TerraUSD (UST) was hyped up and became popular mainly because the network paid 20% APY interest on UST deposits, which is a way higher rate on a "stablecoin" than you can get anywhere else (rates in much safer protocols are about 5-8% right now). Obviously a 20% interest rate is unsustainable when it's supported by money printed from thin air. But people were either unaware of this (unsophisticated investors), chose to overlook it (calculated risk-takers, RIP), or positioned themselves to be able to find a chair right before the music stopped (institutional investors like Celsius).