Constantinople was closing to a near end from its most aggressive neighbor in the east. In Constantinople, it was(Or still is?) the head of the Eastern Orthodox Church. There must have been some form of religious political power it symbolized or represented to all of Europe and even to the Vatican in Rome.
Why wasn't the European Monarchs so eager and focused into preventing the collapsed of the Eastern Orthodox Church? Was Rome more important to them then Constantinople?
There was a similar question recently so I'll link to my answer there - in short, the western church wanted the Greek church to submit to Rome, and they were never able to agree on that in a way that satisfied both sides. Also, the political powers in the west were busy warring against each other, or against religious heretics, so no one was willing to help Constantinople in 1453.