I wonder if they can be trademarked. If not now, maybe later.

It's more difficult to trademark stuff like style, there are certain rules of what can be trademarked; long story short, non-graphical representations need to be clear, precise, self-contained, easily accessible, intelligible, durable and objective.

Trademark also gives a more limited protection in some ways.

Yes they probably can be trademarked.

But a big question is why? Trademarks are very expensive.

It makes sense for a company logo that was AI generated, but spending 100K+ for very limited protection of a piece of art seems questionable.

Trademarks are actually much cheaper than that, I paid a few thousand dollars for mine. The price range your quoting is more typical of patents

You can lose your trademark if you do not actively enforce violations. Hence, the full cost of a trademark should include its legal maintenance.

that is an unknown quantity which one cannot estimate

Was it international? And in how many categories?

It adds up very quickly.