My only guess why the Mexicans did that instead of just making hash is reduced labour.
There is this VICE documentary (I think?) where you can see how they make hash in Morocco. There are the plantations and then labourers keep drumming the weed bushels for hours on end with wooden sticks, on top of a sieve. Then gets pressed into hash.
Mexicans were probably too lazy to do the drumming, so they skipped that part
Still, the Moroccan hash that I know was pretty bad. Also sometimes you would simply get sold a piece of plastic, wasn't exactly easy to distinguish... I've never bought anything off the streets back then anyway; that was in the late 90s. Weed was quite difficult to find, sometimes somebody would bring something from NL, or found a contact who had some. At some point a few years later suddenly weed was everywhere, and nowadays nobody is smoking hash anymore. Moroccan smuggling must have gone way downhill.