I'm on mobile, so I'll need to come back to this with better sources and formatting, but I honestly believe it's a combination of restricted market and France's Italian ambitions.

First, who can "afford" Byzantine titles? Spain is recovering from La Reconquista and looking west, not east. So no go there. England is relatively poor during this period, as the English monarchy is consolidating power and fending off nobles - and is probably not interested anyway. And the Hapsburg monarchy could pay, but why? They're more interested in securing a border with the Ottomans rather than expanding into the Balkans. And Russia doesn't really exist as a kingdom yet. So France is probably the only one to be willing to purchase such titles.

Second, the purchaser of the titles, King Charles VIII, is the man who invades Italy in an attempt to expand French borders in the only direction where he could meet success. While he had a slight claim to the Kingdom of Naples through marriage, possessing titles that might entail him to ancient Roman/ Byzantine land claims could shore up his reasons for invasion. Legitimizing an invasion in multiple ways would help his nobility sign on to his war effort and hopefully increase the chances of success.

I very much doubt it was ever about reconstituting the Byzantine Empire. Western Europe had written off much of the Ottoman European holdings as "lost", so it was just the sad and inglorious end to the Byzantine claimants.

Edit for sources: http://zimmer.csufresno.edu/~mariterel/hist125_fall'03_italian_wars_lecture_notes.htm http://europeanhistory.boisestate.edu/latemiddleages/invasion/invasion.shtml http://www.britannica.com/EBchecked/topic/107175/Charles-VIII