They didn't make the playoffs because they were plagued by injuries. Kucherov had 85 points in 70 games last season.
If that's the argument you're going with, why wouldn't Stamkos be the Hart favorite?
It's not the only argument, nor the overarching one, but I think it provides a bit of context as to why, versus Hall, MacK, and McDavid (who would round out my top 4).
Ultimately, Tampa has been, debatably the best team in the league all year......and Kucherov is the best player on that team IMO, he drives that line more than Stamkos. That's why he would be my vote for MVP.
I don't have a horse in this race, but I think if it were simply a vote for who's the best player, it would be McDavid and it wouldn't be much of a debate.
I don't like getting into hypotheticals either with injuries. I think it's a reasonable assumption, but not an absolute given, they are anything more than a low-seed playoff team last year. Otherwise, the Oilers would have been lock for this year's playoffs as well.
But it should be noted- that a jump (either way) from an injury riddled team that barely missed the playoffs to a legitimate, ~top 2 team all year is a sizeable gap. Probably moreso than the gap from missing the playoffs->wildcard. I don't think you should discredit or overlook that notion (not saying you are).