(Non-pilot speaking)
But if you placed it in the engine - you have to pump the fuel there first ...?
My uneducated guess as to why fuel heaters were discontinued would be:
1) We have better fuel spec these days
2) We can save the weight and cost of these units
Followed by:-
3) It's worked well since we stopped installing fuel heaters ...
Humans usually have to learn things more than once.