Because they are not familiar with it. They won't know what's the difference between zkSNACKs, mine, and any other coordinators there are.
If a user was blacklisted by zkSNACKs then they would recognize the difference because they would not be allowed to participate. Since everyone seems convinced blockchain analysis is a scam that doesn't produce accurate results, you assume there are many of these users who will switch to your coordinator, right?
Besides, you can always outcompete zkSNACKs by charging lower coordinator fees or by offering different round parameters that better fit to certain use cases.
When we are on the subject of alternative coordinators, can you tell us how many there are currently? Also, how does their traffic look like compared to the default one?
I don't know.