I think your time frame might be a bit off. During the 15th century (1400s), the Ottomans were almost continously at war with various Balkan countries (Byzant, Bulgaria, Serbia, ultimatly Hungary, etc.), slowly taking them one by one, piece by piece, so they did expand into Europe. At that time (beginning of 15th century) Ottoman state was consisting of just lands in Anatolia and Balkans, and was slowly starting to expand. Check this map for timeline of expansion

At around that time, Ottomans had several directions they could expand (and they more or less DID go into all of them).
One was taking Vienna and then going to Central Europe, true, but they had strategically, economically, religiously and militarilly better goals.

First target was the Mameluk Sultanate, which controlled Levant, Syria, Arabia and Egypt. Which the Ottomans did conquer by 1520s. This was probably much more important then expanding into Central Europe, with its riches, spice trade and islamic religious centres. It also brought new responsibilites (mainly caused by control of holy sites and divisions in the religion as well as the Portuguese invasion of Indian Ocean and losing the spice trade) and possibilities (possible invasion of Mesopotamia and control of Red Sea, which they did later in the 16th century )

Second direction would be tied to the Black Sea, with conquering Caucaus countires on one side, and connecting to Crimean Tatars (their vassals i believe) through Moldavia and Ukraine on the other side.

Third direction of importance was taking control of the (East) Mediterreanean and its rich trade. Rhodos, Cyprus, Crete, Malta, Tunis & North Africa all of this was very lucrative to take and hold, and Ottomans clashed frequently with Habsburgs ( and other nations ) in Mediterreanean during the whole 16th century

I would actually say all of this territories were objectivly much more valuable to control and hold then entering Central Europe, not that the Ottomans didn't try that either

So by now we are in 1526, Ottomans control Middle East, parts of Black Sea and Mediterreanean, and just won battle of Mohacs and basically won Hungary. Now, even if we forget that the East was probably more important to hold, one could still ask why didn't Ottoman expansion to West continue

One possible anwser, which i personally believe to be true, is that Vienna was actually the most western point Ottomans COULD possibly physically reach (or just outside of that reach)
The reason is that most Ottoman army campaigns (in the West at least) were seasonal. The army would assemble in spring, march up the Danube valley, do its campaigning and battles in Hungary during late summer, autumn, then return before onset of winter. I am not sure if it was mandatory, but it is how their campaigns went.
Lets then show an example of how it went based on two campaigns to take Vienna 1529, and 1532
In 1529 Suleiman went to conquer remains of Hugary outside of his control and possibly take Vienna. In 10th May* 1529, his army went to campaign. Heavy spring rains made the march slow and lose a lot of its siege weapons, so on 6th August it was in Osijek / Esseg (in today Croatia), entry to Hungary. On 18th August it linked up with Hungarian allies on Mohacs, 8th September it took Buda, and in late September (around 27th September) besiged Vienna, which hastly prepared defences. After few weeks of siege, autumn rains, and snow, poor supplies, sickness, onset of winter, and failure to break actually strong resistance made Ottomans try one last attack, and then upon its failure retreat back to Istanbul.

Second attempt of taking Vienna 1532 was better prepared for, but consequently by both sides. This time Suleiman advanced rapidly, and by July 12 was in Osijek (almost full month earlier then in 1529). This rapid advance suprised the Habsburgs who were raising a large relief army from Germany with Spanish troops under command of Emperor Charles V. However the huge Ottoman army became bogged down for almost a month in unsuccessfully trying to capture fort of Guns (Kiseg) against 600 defenders, and by that time a large enough Imperial army was raised, and Ottomans knowing this, and realising they do not have enough time anyway withdrew home. Similiar thing happened in the last capmaing of old and sick Suleiman the Magnificent in 1566 on Vienna. He went on a campaign in May, but became bogged down for a month in for them costly siege of Szigetvár. Just before Szigetvár fell, Suleiman died of old age and sickness, and the army retreated.

Anyway, all this was to show that Vienna was quite distant for Ottoman army to reach in time for a successful siege. And if it did reach, it would face the combined might of the HRE and Habsburgs coming to relieve the city.
However, this same relief army would usually not contribute to retaking the conquered areas which made Ottoman possesions relativly safe, and Habsburg attempts in the 1540s to exploit Ottoman wars with Safavids to retake Hungary were disasters, and forced humiliating treaties for peace, which were very beneficial for Ottomans in a time when they were warring on all sides

After Suleiman, in 1570s Ottomans entered into a financial crysis, caused by decades of costly wars, military spending to keep up with European technology, loss at the battle of Lepanto, but also as a consequence of Portuguese control of Indian ocean, and falling silver prices after Spanish conquest of Americas. While still being very dangerous, it is probable that it lost the edge and advatage the Ottomans had to go deeper into the Central Europe

*I am actually taking most of this dates from wikipedia, but I see no reason know why it would be a bad source in this case

It might be because of Cem Sultan. He was the uncle of Sultan Selim I, who conquered Egypt.

According to some historians (will find examples when I get back home), the reason Selim I did not continue the westward expansions, was because the papacy had a possible pretender which they could support and eventually dethrone him.

Are you talking about Sultan Selim I? Since the expansion of Hungary did happen in 16th century and Ottomans very much carved through Balkans and Central Europe in 15th and 16th century. Selim was the only Sultan who focused solely on the east.