Mongols in the Levant and Beyond

Did you mean why didn't they send sufficient forces? 

The main reason was that when their Great Mongke Khan died, Hulagu had to consolidate his conquest of Persia and Mesopotamia (which became the Il-khanate), So he pulled out most of his men that were heading toward the Mameluks. The small force that remained, along with allied auxillary units, were destroyed by the much larger Mameluk force.

So it was all about politics, basicaly... The Golden Horde Khan in the north disapproved of Hulagu's treatment of the Muslims. After Mongke died, civil war erupted over the entire Mongol Empire. Hulegu went to war with Berke Khan of the Golden Horde's Khan, so military campaigns elsewhere were out of question. When peace resumed and Kublai became Great Khan, some of Hulagu's veterans were moved to campaigns in China.

In 1281, 20 years after Ain Jalut, the Il-Khanate finally sent a large army against the Mameluks but was deated at Homs. It wasn't until Tamerlane that a "nomadic cavalry army" from Persia was able to crush the mameluks.