The direct trade route to East Asia was controlled by other empires.
Because of this, they were forced to find another way in order to not be taxed. On top of that, the way to East Asia was not exactly the easiest as there was no exact direct way for the Europeans to get there - through the Mediterranean was a no go as Greeks lied there as well as the Middle East at the exit. The other way would be around Africa but that would take to long. Thus, they searched for a simpler method.
Hope this helps!
