The primary reason is a geographical one. Acre was a coastal city with a fortified harbour, and so it could easily be resupplied with both food provisions and reinforcements by ships sailing down the Levant coast. The main reason the crusaders were able to capture the city in 1191 is because they gained naval supremacy in the eastern Mediterranean, and so Saladin was unable to send food and supplies into the besieged city, which is what doomed it. A relatively small garrison could successfully defend the city so long as they had ample provisions being shipped in by sea.

This is in contrast to Jerusalem, which is an inland city located among rugged terrain. Defenders aren’t able to have supplies and reinforcements sent in whenever necessary, and because there’s no sea to act as a natural defence of one side, Jerusalem needed to be defended on every side at once. Any relief army needs to march inland, and can easily be intercepted. This is what happened during the Third Crusade. Acre was designed specifically to be easily supplied and easily defended; Jerusalem, built long before conceptions of naval resupply were a thing, was not.

Source: God’s War: A New History of the Crusades by Christopher Tyreman