Strange, I wouldn't expect litecoin to be that popular. Perhaps it is due to the recent high Bitcoin fees, that people went to LTC instead?

As for why they don't use Monero, I'd say it is probably because Monero has an even steeper learning curve than something like Bitcoin. Personally, I am still holding my XMR on the exchange because I've read countless times that the desktop wallet is not very user friendly. I'm waiting for the ledger implementation. I think once user usability in Monero improves, adoption will increase too. It just doesn't make sense to do shady things with BTC/LTC