Apparently there is a cash shortage, which could make it hard to supply all the registers with the right amount of change.
I also would assume that most of the transactions they do are already cashless and that almost every guest is capable of making cashless transactions, which makes it an easy switch that won't upset a lot of people.
They might pay a bit more on transaction fees, but they can get rid of the whole cash infrastructure, which probably makes it worthwhile.
Is theft a big issue in theme parks? By going cashless they could avoid people swinging wallets full of hundreds of $ around, that might be stolen later causing upset guests the park would have to deal with.