Many people were fed up with the Spanish king at that point and invited the Muslims to take over. Spain would not be captured so easily if the inhabitants fought for it instead of against their current rulers.
"The last Visigoth king, Roderick, was not considered a legitimate ruler by all of the inhabitants of the Spanish Kingdom, and some Visigothic nobles aided the Islamic conquest of Spain. One name frequently mentioned is Count Julian of Ceuta who invited Tariq ibn-Ziyad to invade southern Spain because his daughter had been raped by King Roderick. "