This is a theory about all flat worlds. The reason why there are only slimes is (I might be wrong this is a theory) the are happy because there is no water or lava. Creepers aren't there because they are bored since there are no hills to blow up. That is also the case with other monsters. There are no interesting things like wolves,ocelots,squid or bats. Why?...
See in the next theory about all flat worlds.