In my opinion I think modders focus on ELS due to the "pros". A massive quality of life feature that I have always wanted is regarding headlights/takedowns. With GTA 5 if you put on high beams and get out of vehicle even with leaving the vehicle turned on, the high beam light dims and loses the extra range. ELS makes it so this does not occur even with takedowns. ELS can be customized to change the flash patterns to the users liking, same with what siren tones are allowed. Some vehicles before ELS had none or little environmental lighting, It seems that this is very easy to do with ELS as I have yet to see an ELS vehicle with no environmental lighting.
Hopefully someone who does vehicle modding could chime in if ELS vehicles are "easier" to make then non ELS. I say this as the lights are linked to extras, barely any ELS vehicles require carvariation Carcol edits. In saying that some popular vehicle modders have only released very few cars sometimes only 1 since ELS has came out (that could be for many other reasons).