Considering this and the market cap of USDT, the tether official wallet is still unable to gain hype and is a big failure. As far as I know, it could have been a much better option if they had only launched a wallet that supports tether only and not other crypto like BTC, etc.
I don't think that's the reason. Investors are lazy and don't like to download hundreds of wallets. For them, one wallet that supports hundreds of cryptocurrencies is better than 10 secure wallets, and these are the majority who might download Tether.wallet.
I don't agree with this. Sensible people will store bitcoin in a bitcoin-only wallet and not in a wallet that supports multiple cryptos. Besides security reaosn, another reason is that most people have bitcoin as a major cryptocurrency and they shouldn't be invested much in altcoins ( remember sensible people

).
Now, after Bitcoin, I guess the next major crypto that people have is stablecoins. ( Even holding stablecoins is better than holding any garbage altcoins). I think if there was a tether-only wallet by Tether, it would have been more appreciated, downloaded, and used.