Well, it is still kind of a grey area... I know there was a change, and even someone like me who is a legally trained does not know the exact law these days, and in reality, *nothing* has changed! Organised chaos still rules!
As for why there has been an issue, I think it comes down to French culture, and the sheer number of incidents and deaths on bikes. For example, in France you can ride a 50cc on public roads at 14

, a 125 at 16 after a small test, and a 1000+ at 18 if you take 'proper' test. All in all, everyone rides a scooter at one time or another... and they get *very* complacent. I was in Rennes this weekend, and I saw so many 'kids' on scooter and small bikes without any kind of gear, no protection at all in terms of gloves, jackets, even lids in some places! You literally leave your house, jump on your bike in t-shirt, jeans, maybe trainers...
When I met my g/f's mother, she heard I had a bike, and was not happy. She then told me that out of my g/f's class of 30 people at school, 3 of the guys have been killed in bike accidents in the 10 years since she left... it is a sobering thought.
The issue comes when this culture translates to bigger bikes. Yes, I often wear jeans and not proper leggings, but this is the only concession I make. Here you often see GSX-R/Supersport riders without gloves, in shorts, trainers etc. It is an accident waiting to happen... Heck I saw a guy on a duacti 1098 at the weekend in a white linen suit combined with his g/f on the back in a evening dress!! Combined with the fact that the country is not densely populated, you have a recipe for issues. If you crash on a quieter French motorway, it could be a significant amount of time before anyone even finds you! I did 200kms the other night at 2am on the A84 (Normandie/Brittany) and didn't see a car for 150kms! There was also no motorway lighting... I was thinking, god forbid if I came off and went over a barrier, the first person to find me may be a farmer next spring when he ploughs his fields!!
As for specifically lane splitting... generally people are confident, if not overconfident riding bikes as they have done so from 14 onwards, and there are no dedicated splitting lanes... you just filter between the cars and lorries. If you see the 'Periferique' in Paris, really, WOW.
All in all, in France, if there is a law they don't like, they ignore it, and that has been the case with splitting, but the sheer number of deaths and incidents here meant something had to be done
But then of course you get people trying to copy 'The Black Prince'...
YouTube - Le Prince Noir sur le périphérique parisien Part 1
And even 'Ghost Rider' has done the Periferique...