Are Slovak and Czech the only Slavic languages that use "h"? And why?

If you mean the letter "h," then it is used in those Slavic languages that use the Latin alphabet (Slovak, Czech, Polish, etc.), but not in Russian, Ukrainian or Bulgarian (which are all written in Cyrillic). If you mean the sound "h," then Czech, Slovak, Polish and Ukrainian, possibly other languages too, have this sound. Polish has both "h" and "ch" (the latter pronounced as the Spanish "j" or the Scottish or German "ch," therefore it is NOT the same sound as "h," albeit a similar one, particularly to a native English speaker's ear). Russian only has the "ch" sound, but not "h."