Apparently when the stele was discovered, locals broke it up to sell the pieces for more money.
At any rate a collection of ancient texts in translation says that the Mesha stele was broken up by "the Arabs." As if all the Arabs were guilty of the vandalism.
It was found intact, and the finders were Arabs, couldn’t agree on the division, and thought also that they’d all make more if there were more artifacts to sell. So they smashed it.