have an earlier conquest of constantinople. perhaps one done by the Arabs.
Have the subsequent crusades fail miserably, hence no possible land route to the indies, hence an incentive for finding alternative trade routes, that leads to naval expditions financed by the europeans.
Have the reconquesta be completed in advance, so Spain and Portugal have the funds to launch said expeditions, otherwise those funds would be used to bolster their armies at home.
I think that would be enough for the americas discovered in 1392.