I answered this a little over a year ago, and judging by the reply to the top deleted comment, gave a similar answer to his. I'll paste it here anyway, and judgements can be made, and arguments rehashed. That is after all how historical scholarship flourishes.

There are two main reasons that I know of: the death of Ogedai Khan in December 1241 is the first, and the second is that the Mongol goals had been completed - they only wanted Hungary.

Ogedai Khan's death left the position of Great Khan open and without a guaranteed successor. The Mongols didn't adhere to "the eldest son inherits". Ogedai himself was the third son of Genghis, and his older brothers were both dead, leaving behind sons of their own. Because there was no clear rule about succession, it was open. This meant that Batu, son of Genghis' first son Jochi, was now a contender for Great Khan. Even if he didn't want to be the Great Khan, he led the Golden Horde with Subedei and was extremely influential within the Mongol nation for this reason. Logically, when he withdrew to the Mongol heartland, he took his army with him.

The second reason is that the goals of the Mongols had been achieved. Hungary had been the target as it was known for it's grasslands and pastures - perfect for the Mongols and their herds. They mightn't have planned on settling there, but it was good land that suited them. Beyond Hungary lay much rougher terrain that was ill suited for them. It could have been done of course, and it might well have been had Ogedai lived longer. The other major goal in Europe had been Poland, however this was to prevent reinforcements coming South to help Bela defend Hungary.

So the Mongols had the ability to continue west into Europe, but didn't. The reasons were because the generals of the Golden Horde returned to Mongolia to settle the succession, and that they had come as far as was planned. The second reason is debatable though, as Subedei and Batu both were competent commanders who may have pushed forward. This is however, speculative.

Source: Genghis Khan: Life, Death and Resurrection by John Man.

In response to the thesis that /u/Steko posted, I would agree with the theory that fortifications posed a problem to the Mongols, but only in the sheer number that Europe had. The Mongols had taken many fortified cities through their conquest, but the prevalence of castles in Europe was a different matter. It meant that the Mongolian army would eventually be surrounded by fortified encampments if they couldn't take down each one quickly - and they often relied on siege tactics(starvation and waiting out an enemy).

I discussed this in further detail in this answer.

question: Ive recently found out that Serbs somehow managed to defeat a Mongol force... anyone know more about this event ?