Because it's very hard for a non-official language to survive throughout the ages. In order for that, some very specific conditions must be met. The situation in Spain in which several regional languages survived is actually an exception from the norm. In short, the survival of a language is closely tied to its status. A language that is for some reason considered to be prestigious may survive (although even that is not assured) because its speakers tend to use it and pass it down the generations and even some non-natives may have an incentive to learn it. On the contrary, a non-prestigious language may be under severe extinction pressure. It may be for example associated with rural unsophistication and its speakers may conceal it, forget it altogether when they move up the social ladder or/and intentionally raise their children in a more prestigious language in order to improve their chances. A few hundred years ago, Europe spoke in hundreds of languages arranged in several dialectal continua. Nowadays most of these languages are gone, and with them their rich cultures.

Spain was a bit different. It was much more decentralised than most of Europe's monarchies. This decentralisation meant that it was composed of several highly autonomous communities which were largely left to their own devices. There was no single prestigious language, because Castillan, the basis of the modern Spanish language was largely confined to the crown of Castille (its domination overseas is completely separate issue).

In difference to Basque or Catalan however, Mozarabic was not tied to one specific kingdom. Its speakers were scattered in Castille, Aragon or Leon, each of which already had theor own prestigious language. Mosarabic speakers found themselves in the situation of speakers of a non-prestigious provincial dialect, which additionally carries the stigma of subservience to the Moors (as opposed to speakers of Castillan or Leonese who drove the Moors out). Any upward social mobility required the knowledge of the locally-dominant dialect. Any contact with the government required at least passable knowledge of said dialect. It's no surprising then that the speakers of Mosarabic started to learn Castillan or Catalan or Leonese in order to improve their lot. Then they taught that language to their children as their primary language, because what good could have come from them speaking the bumpkin language? The relative closeness of all Iberian language definitely made the shift all the easier.