Because to Arabs the European world was a place to rule and live, not a colony. To the Europeans, the arab countries were but colonies to expand their interest and resources.
And the Arabs holding Spain back is utter crap. Certain people should stick to politics, anybody can argue that. History actually requires knowledge and study, not just semi-thought out comments to suit your view of the modern world. Spain under the Arabs reached a level that at it's height easily surpassed the Byzantine Empire of the time. Constantinople was largely responsible for the rennaisance yes, but it's golden age came long before Cordoba reached hers. Europeans rulers asked for their heirs to be educated at the university of Cordoba. That Spain had some worthy intellectuals before the Arab conquest might be true, but then again much of Europe had a few names here and there. In the long run it didn't amount to much. Considering how science and intellectualism was in Muslim Spain, to say that the Arabs held Spain back would mean saying that without Arab interference Iberia would have been on the same level as Constantinople, something I think many of you would be far less eager to argue.