When Spain was ruled by Muslims, how was it that a Moroccan power base camp to dominate the Maghreb, instead of an Islamic Spanish based empire?
The base of power for Islamic Spain had always been in North Africa instead of being based in Spain itself. This is due to two reasons.
Firstly, the Arab population at the time of the conquest and afterwards continued to be low. There were not enough to support them as in other locations, such as in Iraq or even Iran. Conversion was not really sought after, hence why a large number of Jewish and Christians retained their religion, and those which the state could rely on were low. Hence why it was much better to call people into Spain rather than rely on the Spanish population to help.
The recruiting grounds for Islamic Spain had always been in North Africa. From here you could bring up the most warlike people around, whenever your dynasty was threatened, and they had no loyalty to anyone else except to Islam and to the Caliph who brought them in. Later when the Umayyads fell in Spain, it was local dynasties in North Africa who arose to claim the position continuing the tradition and basing their power in the region, with Spain and important, but not central part.
Secondly, and more specifically to the dynasty you refer to, the Almoravids never held their centre of power in Spain but rather from Morocco. There was never enough political or military force to reverse the flow of conquest from Spain into North Africa primarily, once again, due to the manpower that was available to the North Africans either through tribal loyalty or Islamic religious fervour.
Much like later dynasties, the Almoravids were called into to help the ruling Islamic houses in Spain to prevent the Christian conquest of their cities. Once they had defeated the Spanish armies, and finding the ruling houses weak and divided they took control of Spain and ruled.
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