I've got similar answer when I was attending the binance meetup few months ago .. it's indeed they want a better quality market volume and instead having too much pair with those new stable coins, they choose usdt as the main focus.

I don't know if they have some 'under table' deal with tether ltd.  I won't make any wild speculation here, I don't want to

It's also confusing for the newcomers when there a lot of similar usd stable coin pairs like usdt, tusd, usdc etc. Not really effective to trade with such low volume.. as the volume is separated and for me personally it's hard to take an opportunity to make some profit when there's a low volume.