Bartholomew Diaz, who was sponsored by Portugal, had just rounded the tip of Africa. Ferdinand and Isabella were desperate to find a way to India (Asia) before Portugal. They were looking for a new way to get there faster.
Columbus had already tried to go to Portugal, but, as said before, Diaz had just rounded the Cape of Good Hope, so Portugal didn't want/need a new idea.
Hope this helps! Please let me know if I'm wrong :)
