Lawsuits are expensive, and they continually get more expensive the longer it drags on if the matter doesn't settle. As a result, there needs to be enough money at stake to make the high costs worthwhile. Additionally, you need to have the financial means to afford the legal action which many cryptocurrency fraud victims do not have.
Additionally, the reality with a significant portion of cryptocurrency disputes, in particular matters that involve alleged fraud, is that the victim may not know the real name of the person(s) that defrauded them. Or there may be jurisdictional challenges associated with pursuing someone in other country. Both of these can make it less practical to try and file a lawsuit.
While a lawsuit can be filed against a 'John Doe' with the hopes they can be identified later on, it is important for there to be assets that one can go after if needed. If as a plaintiff you have no idea where any assets are that you are hoping to collect, and don't know who the party is that defrauded you, a lawsuit may not be viable.
See an overview of the pros and cons of taking the civil litigation route vs trying to pursue with the help of law enforcement instead
