Objectively, if used correctly, ZCash has better transaction privacy, at least until FCMP++ goes live with (probably) the next Monero hardfork. After FCMP++ goes live, no, Monero will be the objectively best privacy coin in all aspects.
In reality you have about 400 fully private ZCash transaction per day and 40.000 for Monero.
You might literally be the only person making a Zcash transaction that day within a +100 mile radius, not much of a crowd to hide in.
There are many other factors like ZCassh having a CEO, dev fee, moonboy community and even their garbage tier mascot that make people prefer Monero.