Basically, because he had other more pressing concerns.

To back up a bit - Mehmed’s last campaign was a two parter, one part Otranto, which succeeded, the second part being the siege of Rhodes, then held by the Knights of St. John. Although Otranto had fallen and provided a large haul of slaves and other booty to the empire, the Knights held firm and at significant cost to the Ottomans, who had to give up in August 1480. (Guillaume Caoursin wrote the standard contemporary account; Eric Brockman’s Two Sieges of Rhodes, 1480-1522 is a good modern account)

Holding on to Otranto over the winter 1480-1481 proved a little troublesome as well. Lack of food. The commander withdrew a large part of the force to Albania, with plans to return the following spring (1481) to try again.

In the meantime, the Pope managed to convince other European powers to join together to confront the Ottomans. Not quite the full scale crusade he would have liked, but an imposing show of solidarity nonetheless, and with Otranto as their first order of business.

Then, on May 3, 1481, just as the season of war usually began, Mehmed died.

Which gets us to your question. Succession was not smooth. Mehmed’s elder son Bayazid, by all accounts a peaceable scholarly soul, was the logical choice, but Bayazid’s younger half brother Djem had his own idea and attempted to use Egyptian Mamluks to take power for himself. Bayazid had to deal with his brother before anything else, one aspect of which included making a deal with the pope to leave Otranto. By the time Djem and his army was defeated (Yenişehir, June 20, 1482), any lingering momentum for a fight with the west was gone.

The aftermath was bizarre. Djem, having lost the fight, took refuge with the Knights of St John, essentially on the grounds that the enemy of my enemy is my friend. He later wound up in Rome under the protection of the pope. Bayazid, concerned that Djem might make an alliance with the Christian west and together take back Constantinople (the city’s shift into Muslim hands, 1453, was within living memory), paid the Knights and later the Vatican a yearly stipend (45,000 ducats) to keep his brother isolated and out of mischief. (Djem eventually died in a Naples prison.)

Bayazid was succeeded by Selim the Grim, another expansionist sultan, who was in turn succeeded by Suleiman the Magnificent, who at the outset of his long reign managed to expel the knights form Rhodes, and at the end of said reign (1565) failed to expel them from the island of Malta. But that’s another story which I’ve written about extensively elsewhere.