So now it's review copies that are the "problem"?  That dastardly Nintendo, doing with the Switch and its games what every other company does with every other console and games.

Or maybe you, sir, are in the minority and shouldn't have the profound arrogance to declare your opinion to be law and accuse the huge numbers who disagree of conspiracy and impropriety. And if you are trying to use the user score aggregate on Metacritic to represent all gamer impressions, I say you should take some time to reflect on what you are doing.

And then you say there is no reason and then proceed to give a reason and even explain how it is a good reason.  I mean...what?  And then on top of that, forget that different individuals may be reviewing the game this time around.  

You should consider that the 88 was obtained by the game without the DLC content (and without the 200cc mode). If MK8 WiiU had have those extras tracks and characters it could have scored better.

It also comes with the DLC, which obviously didnt have when the game launched.Then there is also the new dual items system, which can change the gameplay and strategies considerably.There is also the slight improvement in graphical quality, and there is the portability.I would say all this warrant the jump in score.

I'm guessing this a joke ...at least I hope it is.

It could be, but it's not something that critics are explicitally pointing out, from what I reading.

If the original release didn't get a 90+ Metascore it's because of those 75 and 80s it got. Checking out Nintendo World Report's reviews for both the Wii U (7.5) and Switch (8.5) versions we can see that the "miserable" Battle Mode was an important factor for 7.5. Now that the Switch version has an improved mode, it obviously should have faired better.

Other example is Gamespot, which also complained about the Battle Mode and gave originally the game an 8 score, now it awarded a 9.

EDIT: GamesRadar, this one is huge. Originally 8 and now 10, but for some reason I can't see their 2014 review anymore, seems to me they just updated it.

If what you say holds true then the highest scoring games of all time should have at least a couple of handheld games in there since they have a higher chance of scoring high scores because of their "simplicity". 

Let's check metacritic:  On the top 50 games of all time according to Metacritic we have one handheld game. A Link to the Past GBA on the 49th spot.

I just don't see handheld games "simplicity" as a reason for the higher score. Portability and a better battle mode are more logical conclusions.

Great score, better than I expected!

Well first off, the whole portable thing will almost certainly affect scores in a variety of ways depending on the reviewer... where some marvel at what's been pulled off as a portable console, others dock it points for being less capable as a home console.

Second, I remember we discussed it a couple days ago and you suggested there would be a "remaster" effect, causing reviews to drop a few points as it's not entirely new software. I did see that has happened in a couple of reviews... for instance the current low "80" score said the game was excellent but docked it 20 points as people who owned it and spent ~$100 on DLC wouldn't get much out of this experience that they haven't already.

That "remaster" effect you speak of isn't a given, though; The Last of Us scored a 95 on the PS4 just as it did on the PS3, and the upgraded PS4/Xbox1 GTA V also maintained the 97 that it previously had. For the most part, then, games that were already scoring very high tend not to see much fall off in their scores when remastered, and given how crazy high the two I just mentioned had already scored, it's really saying something that they managed to reach those heights again.

Mario Kart 8, meanwhile, had more room for improvement at merely 88. Unlike those others, this really has offered more in terms of upgrades when you consider what it's bringing to the table; its resolution has been improved, it's playable while portable now, both modes hold consistent frame rates without stutter, the huge negative of the original has been fixed by including five new battle maps and multiple battle modes, and it includes some ~$100 in DLC with additional maps (basically meaning it comes with 48 courses rather than 32, excluding the 5 battle maps).

Chances are it'll still lose another point or two as more reviews come in, but given all of this it shouldn't come as a surprise that this might gain a few points in the current review.

Oh please, can we stop this nonsense about reviewers hating on Nintendo, it's ridiculous. It's getting higher reviews because the gameplay hasn't aged at all and it comes with far more content than the original.

Nintendo first party games have scored highly on every single console and reviewers aren't out to get them or influenced by the game being on a shit console

I know people get angry when Nintendo games review well, but I never expected to see such salt about a much improved MK8 getting a few more points than its predecessor. It's just weird O_o