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.