Well it's not a Zcash fork but an independent development – it actually launched before Zcash.

Zcash (& all its forks) has the problem of lacking supply auditability, meaning if any malicious person ever finds a bug that allows for double spends or anything like that, they could create an infinite amount of currency without it being detectable. Zcoin doesn't have that problem because it shows transaction amounts, even in zerocoin spends (Zcoin's equivalent to shielded transactions, roughly speaking).

Also this year or next year the trusted setup, which is still at the core of every zerocoin/zerocash-based cryptocurrency, will be ditched in favor of the trustless Sigma protocol in Zcoin.