This is a big question for me too! Especially since they didn't seem to have any major reason to burn a lot of cash or to run out of money! Was it maybe a lawsuit, fraud or regulatory issue? I would LOVE to know.
their own statement:
The drop in hash rate (over 50%) after the last Monero hard fork hit us hard. So did the “crash“ of the crypto currency market with the value of XMR depreciating over 85% within a year. This and the announced hard fork and algorithm update of the Monero network on March 9 has lead us to the conclusion that we need to discontinue Coinhive.
https://coinhive.com/blog/en/discontinuation-of-coinhive (you probably need to whitelist that in your adblocker to read it)
Sounds like "lots of work to do soon and not making all that much money, not worth it". (surely being known as "malware" didn't help either...)