"Is this floor dry?"

Translation: Czy ta podłoga jest sucha?

[siebenneun]

czy ta podłoga to sucha?

why doesn't to work here?

[Jellei]

"to" only works as "is" if there are noun phrases both on the left and right side of it. And on the right we have an adjective here.

You can read more about it here, and Part 3 is the one applicable for this specific sentence.