this idea that becaue AD assassins -> Armpen therefore AP assassins -> MRPen is flawed .
There is no correlation.
The reason AD assassins like Armor Pen is because unlike MR, Armor is more abundant and prevalent. Champs get more Armor per Level than MR and there is more Armor in the game. There is also comparatively less AD in the game, and Armor reduction is less efficient, only affecting bonus armor.
AP assassins scale with AP. There is less MR in the game, champs get less MR per level than Armor and there is comparatively far more AP in the game than AD, Magic Resist Reduction is far more efficient since 
affects total MR.
As a result, Flat MR pen is far less abundant because simply put, there isn't much need of it.
The goal of Flat Armor Pen, now known as lethality, is to get squishy targets who don't build much if any armor closer to 0 Armor.
Likewise, the point of Flat MR Pen is similar, however because there is less MR in the game on top of champs getting less per level, as well as there being far more AP in the game, there is less MR pen in the game to compensate. Having more would quite frankly be broken as you would easily have champions MR in the theoretical negatives.
The reason Lethality is so strong right now is simply a combination of the main difference between The two stats (AD and AP) as well as the current state of the game. Gametimes are super low right now. Most games are decided by the 20-30 minute mark.
When Armor pen was changed to Lethality, it was made to where you don't get a full amount of the stat you purchase upfront. lethality quite simply is Armor pen that scales with the targets level. This was done to lower snowballing.
Upon release, lethality was weak because it gave you 40% upfront and 60% over the course of the game.
Because games ended so fast, you typically didn't even get the full stats of the item at all since games ended by 30 minutes.
You would at best get 60%.
Lethality was recently buffed to where you get 60% upfront now.
This is huge because now to where you would have 60% at 30 minutes, you get 60% upfront and can likely get 80% by 30 minutes now.
It was basically a 20% power increase to lethality across the board.
This isn't to say Lethality is completely and utterly the best AD stat. Technically, it's prevalence is mostly chocked up to the game being centered around the early to mid game eras, and thus late game builds with crit and lifesteal and such don't have as much punch as early as lethality does.
This makes lethality super popular because it allows champions to get stronger powerspikes earlier on in the game than they otherwise would.
I fully expect Lethality to be adjusted to compensate for this. Riot will likely either tone tone the flat values on the items, or adjust lethality to 50:50 this time.
Only time will tell.
(and some other champions 
) are taking full advantage of the "triple lethality build" consisting of 
and probably buying 
at some point.
tend to focus on instead building massive amounts of AP instead of stacking flat magic penetration since the only viable item source of flat magic penetration for these champions is 
, and none of these champions are particularly strong at the moment.
and 
, but those items simply amplify the burst an AP assassin has.
