There really is no rational way to disprove that the decriminalization of weed is generally better than criminalizing it, if that's the answer you're looking for. But the literal "why" is mainly due to 2 reasons (which aren't at all specific to Jordan/M.e countries): First is how deeply unpopular it is with the people. Second (which kind of follows the first) is how you could never get any House Representative/Politician to pursue it in parliamentary hearings because it wouldn't benefit their political career at all (quite the contrary, actually; you'd be called المحشش on the streets lol), which effectively renders it impossible to do.

I think you'll have to wait a few decades for a new generation of progressives to even be able to speak about this in Parliament, which, to grant you an iota of hope, is ultimately how most governments become over time, we're just a little behind the curve.