No, the once liquid, rotating core of Mars cooled and solidified.
How did that happen? Why hasn't it happened to Earth?
Why did it happen to Mars? I doubt that anyone knows for sure, but probably has something to do with the fact that Mars is relatively small and therefore had a smaller liquid core, and smaller things tend to cool faster than bigger things.
But Mercury still has a magnetic field and is smaller than Mars.
Mercury's magnetic field is about 1.1% as strong as the Earth's. What's your point?
It's also strong enough to deflect solar winds, so 1.1% must count for something.
My point is that everyone here is talking as if there's a a definitive answer, and it's clear the best everyone can do is guess. The same thing FErs get criticized constantly for.
The difference being that there are scientists who speak much more substantially, in terms of both evidence and theory, than we lay people do. There is no equivalent that I have seen for FEers, yet FEers often claim to be on equal footing.