How could Ottomans fail to conquer Egypt?
Giving their clear strategical superiority in early XVIth century, I suppose that it means we may need an earlier PoD but it would be best IMO if we consider one that take place after the conquest of Constantinople.
I think we may have some possibilites, ranging from a possible Ottoman dynastical instability, stronger Savafids, and (while I'm not sure how too) Mameluks undergoing a quick reform.
Everything goes as long as Egypt remains outside the Ottoman Empire (Holy Places are free for the taking however), and you don't have to preserve Mameluk rule.
If we can imagine a leader (maybe one of the Mamluk beys, does not matter) who manages to consolidate the power and modernizes Egyptian army along the lines similar to those of the Ottomans then it is a toss. The Mamluks eventually had been defeated by being mounted archers fighting an army that had strong infantry with the firearms and a reasonably good cavalry (I'm not sure if at that point Sipahi had the firearms or if the firearms had been of any significant importance to them even at the later time: they were definitely not a well-drilled "shooting cavalry" of the Western Europe). So the "only" thing you need to add (providing the Mamluks kept under a tight control) is a good native infantry with the firearms and a competent leadership. Such a trifle.
What would happened from there? It seems that decline of Egypt was more or less not quickly recoverable with the spice trade being hijacked by Portuguese, but was it this of a doom for whoever would rule from Egypt?
IIRC, they were still good in the textiles and grain production and while Portuguese cut into the spice trade the African connections were not damaged.
"There is no doubt that Egypt was the biggest province of the Empire with regard to revenue potential, due to its productivity and its unique position between East and West. The products from the fertile lands around the Nile and the coastal area of the Mediterranean served as the main source of food production for the people in Istanbul and Haremeyn - the holy cities Mekke and Medine - besides Egypt‟s people. Its geographic position allowed both the local merchants and administrators to acquire a generous income. Rather than supporting the central government in military aspects, Egypt provided large quantities of grain and tax revenue for the central government . The Ottoman Empire had a huge economic and financial advantage with the acquisition of Egypt in the sixteenth century. Land tax revenues, urban taxes and custom dues from the ports of the Nile, Red Sea and Mediterranean filled the provincial treasury. After covering the provincial expenses as well as the expenses of the annual hajj caravan, grain and financial supports for the people of Haremeyn, the surplus of the treasury was sent to the central government in Istanbul."
http://www.rosetta.bham.ac.uk/issue15supp/gunes.pdf
Well, actually author of the quote above argues that Egypt gained economically from becoming a part of the Ottoman Empire by getting access to its markets but wouldn't the Ottoman buy these products anyway? Anyway, there were other valuable items besides the spices and I'm not sure that the Portuguese cut the spice trade off completely: were they selling spices to the Muslim countries as well?