* FridgeLogic: Ask yourself why Strange felt it was necessary to channel Zom to beat Hulk when Strange on his own is capable of fighting beings that FAR outclass Hulk in every way.
** FridgeBrilliance: There was no other way to convincingly have Strange lose than to have him distracted with something even worse than Hulk during the fight (not to suggest that Strange's defeat was convincing, just more convincing than having Hulk win on his own merits). Still doesn't explain how he allowed himself to be put into a situation where summoning Zom seemed like a good idea, but uhh...what are you gonna do?
*** Outside of Hulk's supporting cast/family Strange has the most insight of any other hero on the nature of Hulk and Banner, having spent so much time with him on the Defenders. He let his guard down when Hulk turned back into Banner because he could not conceive that Banner was working with Hulk. Essentially Hulk beat Strange the same way Strange tends to beat many of his godlike multiverse-threatening opponents. Hulk tricked Strange.
The shattered hands, and disbelief over what happened could presumably panic Strange enough to make a rash decision like summoning Zom (Without his hands Strange couldn't pull off his normal spells, and his mind was too clouded by pain/confusion to really think things through.) Lastly, Strange only channeled some of Zom's power, and in a purely physical manner.