Why didn't Constantinople have cannons/gunpowder during the Fall of Constantinople?

They did. The Byzantine Empire was acquiring cannon in the late 14th century, and cannons were being used in defence of Constantinople against the Ottomans already in the 14th century (1396).

In the siege of 1422, both sides had gunpowder artillery. The siege is described by John Kananos, who comments on the artillery of both sides, and notes that the Ottomans built field fortifications to protect their guns from counter-battery fire by the Byzantine cannons. Kananos wrote that the Ottomans had expected that their cannons would be able to breach the walls, but despite repeated hits, they failed.

In 1453, the Ottomans brought more cannons, and more powerful cannons. The defenders still had guns, but the attackers had more and better guns. Byzantine counter-battery fire was effective against unprotected Ottoman artillery, but, again, the Ottoman guns were protected by field fortifications against such counter-battery fire. Byzantine counter-battery fire was limited by a shortage of gunpowder. There were also concerns that recoil from the Byzantine guns was damaging their own walls (the walls had not been designed as platforms for gunpowder artillery). Adding to such difficulties, the largest Byzantine cannon exploded. The difficulties faced by the Byzantines are described in the account of the siege by Leonard of Chios.