The reason why it grew soooo much was the pandemic. During the early days of pandemic people wanted to get out of crypto and they still didn't wanted to take all their cash out to their bank accounts, that wasn't something ideal for them, so what they ended up doing was move their money from crypto to usdt which made it a bit more difficult because there wasn't enough usdt, there was only 4.6 billion and honestly crypto market is so much bigger, they were a bit late but not too late and increased it to 11 billion dollars, which people just bought it all because most still wanted out.

Obviously this is not sustainable and usdt will eventually go down and people will sell their usdt for usd and the number will go down one day but until that moment this really works for people.