Just try to say Simcoin quickly 10 times in a row and then try the same with Cashcoin. You'll see the difference.

Besides, it's just too confusing because it links two well-known items that belong to the same group - cash and coins.

When you use "coin" separately, it is now firmly owned by "cryptocurrency" (or will be owned, anyway).

When you add "cash" to it, you switch back to the original meaning and the whole name becomes "whaaaa?".