I missed this thread the first time around, so apologies for the delay.
However, my understanding is at the time the Hastings Line was upgraded, there just wasn't the economic case to upgrade the Marshlink. Indeed, having survived Beeching simply by virtue of the surrounding road network being awful (a situation which remains true to this day), I get the impression BR wanted to do as little as possible with it, in the hope it would just go away. Aside from the two endpoints, the only other stop of any significance is Rye, and possibly Ham Street. It was only really in 2002, when Southern had taken over, that investment was talked about. I believe electrification was seriously discussed in the same manner as Hastings - to get rid of the old Class 206 rolling stock - but was rejected in cost grounds, so the Class 371s were installed instead. For the subsequent 25 years, people keep talking about electrifying the Marshlink again, and again and again (it was a favourite soundbite of Amber Rudd, then MP for Hastings) but I seriously wouldn't hold your breath.
Electrification would mean that Southeastern could run 375s past Ore up to Ashford, looping back round to Tonbridge, in an analogous manner to domestic HS1 services that loop around Ebbsfleet via Ashford, Thanet and Faversham. I've seen enough instances of a class 371 waiting at Platform 1 at Ashford right next to a class 375 going in the opposite direction to know the demand is there, and the track layout permits this. You'd get lots more services for not much more rolling stock.
Nowadays, the demand for the Marshlink is clearly there. I'm probably going to be using it this bank holiday weekend and given the weather I expect all daytime services to be absolutely rammed, standing room only all the way.