They were not in a position to. The Byzantine Empire was fragile by this point, and the West had fully removed itself from the politics of the East. Crusading culture was falling by the wayside; crusades had been called against the Ottomans throughout the 14th and 15th centuries but interest was low, and the outcomes barely changed anything (most ended in defeat). The Western Chruch had forgotten that the Byzantine Empire was the bulwark which had protected them from Muslim forces before this time and by 1453 nobody had an interest in protecting a failing empire. Of course, there was a renewed Christian fervor after the fall of Constantinople but by the mid-1500s most European rulers had conceded to a Muslim presence in Europe.