Monero uses CryptNote's ring signature approach, which scales linearly with the number of coins you want to mix with. Want to fully mix 1,000 coins together? You need a 30kb transaction[0]. You chunk those coins into smaller mixing sets, but then they aren't fully mixed. In anything using this approach, your anonymity set is limited by what you can transmit across the network in any given transaction or a small set of transactions. I've never seen an exact proposal for mixing tx size and I'd be very interested to see one, but if it was more than 100 coins per tx I'd be surprised.
In ZCash, transactions are constant size and are fully mixed with every other coin in the current anonymity set.
Both approaches do have the indefinitely growing list of spent tokens issue. Which in practice means you need to move coins into a new anonymity set after e.g. 2^32 serial numbers and throw away the old coins and spent serial number list[1]. So there is an inherent limit on the maximal anonymity set you get out of any anonymous ecash scheme. Zerocash hits that limit. Due to its per transaction scaling issues, CryptoNote simply can't.
As a result, in ZCash, your coin is hidden amongst all the coins in the maximal anonymity set. In Cryptonote/Monero, it's hidden amongst a far smaller fraction of that set. In Monero, you are far less anonymous. All things being equal, you want to be more anonymous.
Of course, all other things are not equal. There are merits to both Zerocash and CryptNote on a technical level, but scalability isn't where CryptNote shines.
[0] Assume one group element per signature in the ring at 32 bytes per element. The real scheme is likely worse.
[1] There more sophisticated approaches that can be used.
