Well the reason why it got popular again is because they introduced new regulations that would be attractive to other potential OEMs.
WEC was at it's peak with LMP1 when those cars were faster in corners than F1 cars but VW group shot the manufacturing costs up in the space and completely ruined it for everybody as FIA/ACO was scared to do anything about it. Toyota's budget was 50 million while the budget for whole VW group with its two "manufacturers" was between 200-300 million depending on the season.
LMP1 specs were the right way to go but again FIA/ACO didn't know how to position themselves as flagship racing series and co-exist with F1. They were threatened through FIA by F1 executives on regular basis as active F1 drivers were showing more and more interest in LMP1. Bottom line is FIA/ACO didn't know what to do with it, instead of looking at how SuperGT dealt with it's GT world dominance as the fastest series in the world they backtracked on the performance citing cost reasons.
Today WEC is GT marketing fad unfortunately just like how F1 was manufacturer marketing fad in early 2000s, believe me. As soon as that fad goes away manufacturers will be pulling out one by one and within two seasons there would be no one on the grid. As soon as one tiny screw goes loose the whole thing will implode and that tiny screw could be relationship between FIA and ACO.
Look at it this way, they still haven't done their homework. They let Nissan do some stupid reverse concept in LMP1 era instead letting them know they can't play like that and that they have to play like others. Today they have wingless Peugeot which is the exact same thing Nissan did a decade ago. They haven't warned Peugeot they can't do that, instead they let them roll with their flawed concept which obviously doesn't work and Peugeot will be the first one to pull out. WEC is gambling on Toyota and Ferrari long term, lets see if it works out.
WRC on the other hand is euro-trash carnival. People who attend these events go there to get wasted, do some forest bbq, get into fights and all other manly things. It's like NASCAR crowd but worse. FIA has been trying to repackage that into something fancy and 30 years later they still haven't. Only the cars got smaller and smaller, at this point they better race go-carts instead.