Explanation: Rome's Spanish holdings were very lucrative - and very contentious. One of the recurring issues was the stubbornness of the native Iberian tribes - many of them were remote mountainfolk or seminomadic herders who had long-established traditions of raiding and feuding, so these strange foreign 'Roman' fellows coming in were not really much of an incentive to stop. If anything, they were fantastic new targets!
Rome's occupation of Spain lasted from the First Punic War to the ascension of the first Emperor, Augustus - a period of nearly 300 years! While Roman volunteers were often plentiful for foreign campaigns, Spain became notoriously hard to gather troops for, especially in the 2nd century BCE. In that time, Spain gained a reputation as something of a miserable posting - where ambushes and enemies were common, but loot and glory were sorely lacking. Roman alliances with the local tribes were constantly changing, as tribes proved themselves as more-or-less reliable (usually 'less'), and only after a prolonged period of warfare, negotiation, and outright butchery did peace come to the peninsula.
For reference, Gaul, itself no stranger to warrior tribes or Roman resistance, became a quiet province in under 50 years. Damn Iberian mountainfolk!