While the M3 is lightly armed and armored the truck would have neither of that so it would be a free kill to any fighter.
So is the M3, despite of armor and gun. Survival chances for M3's are very slim once spotted and engaged.
In a combat zone (troops and town supps), speed is everything. You don't fight your way in, you sneak in, as fast as possible. That's why you also almost never see any SdKfz in this role, even though they are more sturdy than the m3. Carrying twice the load would make the truck much more faster / efficient than the M3.