Assassins and mesmers, why are they so popular now?

Derv and ranger i would put into that also and the answer is simple:

There are no balanxe patches and these are the 4 most played professions cause they most needed

So For mesmers in January of 2012 especially they became one of the best classes for hard mode. In that month to sort of nerf elementalists I think was their reasoning they increased the armor on mobs in hm while lowering their health. This naturally made the armor ignoring damage mesmers can provide better. But on top of that in May of 2010 they were heavily reworked Mesmer bringing them from maybe having a hero to do some god tier interrupts into something that was relevant in damage as well. Obviously PvP always had more a use for mesmers due to their interupts but were niche at best in PvE so this itself really brought them to the forefront. These days to add onto their use in general PvE some speedclears such as DoA have used mesmers for the past number of years (Trenchway existed back in what 2010?).

As for assassins it's pretty similar story I think it was early 2008 though where there was a fairly major skill rebalance that made it relatively easy to perma Shadow Form which led to them being used in most Speedclears. I think that was really the main thing for assassins there have been other changes that made them better a bit in PvE such as even the increased hero capacity meaning you can spare more room for melee support like strength of honor but half the time when people didn't play them in the past it was more about not wanting to be "That assassin." Now I feel that stigma has kinda gone away. Also, assassins with vabbian armor are clearly superior to those in Obby armor.

Also these dates could be all wrong but I think they're correct.

Quote from Marty Silverblade

You've got that backwards. Armor was lowered and health was increased, which had a net effect of zero on armor respecting damage dealers and a negative effect on armor ignoring ones. It was necessary because the 2010 mesmer update made them gamebreakingly overpowered (which is still the case after 2012), but armor ignoring damage had largely been preferred prior to that anyway (see: discord).

The same thing is true for assassins in general pve (I can't speak for farming/speedclearing) - weapon users deal damage by stacking buffs and then triggering them as quickly as possible. The standard dagger chain allows assassins to attack at an obscene rate, and is capped off by the also gamebreakingly overpowered death blossom. This is evident with every weapon user barring dervs often being better off dagger spamming than using their native weapons.

Oh shoot you're right guess my memory isn't as good as I thought sorry.