"An entire legion of my best troops await them!"

The Emperor completely ignored the wisdom to be gleaned from 'The Art of War' by Sun Tzu.

-The Empire underestimated the Rebels, especially their newfound Ewok allies

-Their supply lines got disrupted, making it harder to mobilize their troops

-While having plenty of soldiers and weapons, they split their forces up one too many

-Endor is an extremely harsh environment with unique dangers at every turn

-Most of their manpower got stuck fighting in narrow paths, eliminating their numbers advantage

-Close combat on the surface prevented direct [space] naval support (Mahanian Doctrine)

-Conventional blaster rifles without proper earthworks (entrenchment) or fortifications is useless

~~/o/

Beaten by wicket and the ewoks, no less.