Why are only Clostridium and Bacillus genus able to form endospores? or why aren't all gram negative and most gram positive able to do so ?

Clostridium and bacillus both belong to the bacterial phylum Firmicute, in which some of the members uniquely possess sporulation genes that enable endospore formation. Phylogenetically firmicutes are believed to have diverged from other phyla fairly early, which may contribute to their unique sporulation capabilities.

Many more firmicutes are sporulating, You probably confuse the classes of Clostridia and Bacilli with the respective genus. The other classes with known spore formers are Erysipelotrichia and Negativicutes. In each of these classes there are multiple spore-forming genera.

I am not sure whether the last class, Thermolithobacteria, have spore-forming members, though.