Well, if you
steal it, it's criminal, but that's simply because that's the definition of theft.
Taking stuff away from people, though, is not necessarily theft, and also not necessarily criminal. If you accidentally take someone else's property because you confused it with your own, for example, that's not criminal, but the other party still has a civil claim against you (namely, to be given back their property).
On the other hand, if you intentionally mislead someone into providing you with some service or product, promising to pay them for it, even though you never intended to pay, that constitutes fraud and is very much criminal.
Generally, it's criminal if it's in the interest of the public and civil if it is primarily in the interest of some party. Not paying some debt because you actually have doubts that you have to pay, or due to an honest mistake is not really something that affects anyone else. Someone intentionally causing situatons where others can't rely on them fulfilling their legal oblications can erode trust in a society, therefore it is in the public interest to prevent that. The boundaries can be fuzzy, but wage theft can very much be criminal.
