They did try but western Europe wasn't very enthusiastic about it. There were Venetian and Genoese ships there, as you said, but there were a lot of other things going on in the west to distract the various countries that could have helped. For the church, help was also contingent on the Greek church recognizing the supremacy of Rome, and most Greeks weren't too interested in that.

I answered a similar question in the past, which will hopefully be helpful here:

As the surviving portion of the Roman Empire, the Byzantines were very important in Medieval European thought. When it became clear that Constantinople was going to fall to the Ottomans, did anyone in Europe propose helping them?