It has nothing to do with the wing mounted engines and everything to do with the fact that the wing is very thin due to MDD making it as efficient as possible and also stretching it as well.
The problem is that during cruise the wing cools and can "cold soak" the fuel to temperatures below freezing. During decent into more humid air this causes moisture to sublimate (I think that's the one) water vapor to a solid. Causing ice buildup on the top of the wing. There is an optional tank heating blanket that AA
has, and some airlines have come up with other ways of finding ice accumulation such as strings and "rumble strips" that are smoothed out by ice accumulation. This isn't just possible in cold climates, it's actually more common on days where it's humid and above freezing. I've seen MD
-80s at STL
getting deiced when it was 70 on the ground. A professor I had was an MD
-80 captain and taught systems for a now defunct airline before coming to my school said that he once had to deice at PHX
when it was nearly 90 on the ramp.
The strakes on the front fuselage, however, are heated with bleed air because they found that if ice accumulated on those the airflow would sent it directly into the intake when it broke off.