Not only they could, but they were going to : Isabella's wills clearly states that Castille had to expand on both sides of the Mediterranean Sea, would it be only because IOTL Morroco was once part of Mauretania Tingitana that was itself part of the roman diocese of Spain.
You had other reasons:
- Crush Arabo-Berber control of western African trade roads, especially for the Sudanese gold that represented a huge part of the metallicorigin of European monetary
- Prevent any, more or less fantasmed, possibility of re-reconquista by Arabo-Berbers
- Crush Arabo-Berber piracy (it's why Spain conquered cities on North Africa coast up to Tunisia IOTL)
- Ensure coastal points for trade with southern Africa and Asia.
- It's true that, rather than ending slavery, Christians would takeover slavery trade roads for their own profit as they did in Mediterranean basin IOTL (basically, slave trade roads of XVth Castille are roughly the same than the Arabo-Berber's)
Of course, while all of that is quite plausible, it was soon more or less neglected in favour of Americas that were much, much more attractive.