The liquidity is almost entirely on the Bitcoin side now. Who's dumping Bitcoin for altcoins?
I often look at volumes/reserves. It's common to see XMR (and LTC in a lesser degree) at 0 while BTC is available.
Personally I understand why people trade BTC for XMR, as since last year I'm one of those who reduce continuously their BTC bag and increase their XMR one over time.