>>2058055EMUs have a bunch of drawbacks. For a start, you have to make parts small enough to fit under over over passengers. This can be tricky with really powerful transformers as you'd want them here. Packaging all the other stuff also isn't trivial. It's not just space but also weight, noise, vibration and heat that you need to mange. All this costs extra, causes complications with the coach systems (eg. tanks for the toilet, AC gotta be somewhere too, you'd want huge bogies for high speed comfort...) and makes maintenance a nightmare.
Concerning maintenance, it's also possible to just uncouple a locomotive from the TGV and replace it with a reserve unit if something major breaks. No need to put aside the whole train for days or even weeks. Long term you can also reconfigure trainsets more freely, as it's been happening to the ICE1 fleet lately, simply because most cars in the consist don't have any task besides holding passengers. As for the TGV, having separate locomotives made it easier to derive single and double deck versions as well as multiple generations of locomotives, simply because no complete redesign was needed. Hell, they ran TGV coaches with ICE2 locomotives once.
Coaches that just don't have anything else going tend to be much nicer for customers because it's quite and there's less vibration.
Nowadays they might go EMU for better acceleration but with the technology of its time, an EMU with similar power would've been pretty aweful in every metric.
