how come the mongols didn't conquer europe like they planned?

The correct answer as given by someone else is the Khan died and in Mongol tradition all the tribes had to gather to select a new Khan - there being no right of succession. This took several years and during this period there were some rebellions even among the Mongols that had to be put down. If memory serves well the new Khan didn't last long either so they had to re-run the entire process .

The Great Khan Ogedei died in 1241, just as the Mongols stood at the gates of Vienna and were about to push on to the Atlantic. Needing to be back home, the armies left.

Why they never returned is unclear. Possibly they meant to; but their next major campaign was into the Levant; where both the crusaders and the arabs tried to recruit them as allies. They disdained both; but were heavily defeated by the Mamluks under Baibars at the battle of Ain Jalut in 1260, which seems to have signalled the end of their ambitions in he West.

Edit: I believe Braiden may be thinking of Leo I, who treated with the Huns, not the Mongols.

I never knew they planned to conquer Europe, they probably ran out of military might and great leadership, Im sure it probably is the fact they conquered India through Afghanistan and settled there as it was a land of riches, their descendants are said to be the Mughals who were great warriors, poets, and architectures, They built the Taj Mahal etc rules modern day Pakistan, India, Bangladesh etc

I actually just read yesterday that a Pope, not sure the name, convinced them to not invade Europe, look that up and I am sure you'll find your answer. google: pope who stopped the mongols