Question

why did western european countries in the 15th century feel they needed to develop new trade routes

Answer 1

15th century was a time when the Byzantine Empire fell - in 1453.  This means that the trade routes to India that European Christians were using until this point were suddenly under Muslim control and therefore neither no longer accessible or became a lot more expensive.

This brought them to develop new trade routes. 

Answer 2

B. The Ottoman Empire and Venice controlled existing trade routes to Asia and made European merchants pay taxes.