I don't think they've totally gotten easier, I think that for a variety of different reasons I'm just better at games than I was years ago. I've seen these mechanics many times, I've played significantly harder games, I've recognized the cliches that I'll see time and time again. Like, Super Mario World was a challenge for me back in the day but if it was released now, I would likely fly through it with little issue.
To an extent, I think people are also overestimating the difficulty of old Nintendo games. There used to be the phrase "Nintendo hard" referring to how difficult a lot of games on Nintendo systems were, but a lot of the time people weren't actually talking about games by Nintendo. It was mostly stuff like Ninja Gaiden, Megaman, Contra, Castlevania, Ghouls and Ghosts. Stuff on Nintendo systems, but not Nintendo games.
But I do think Nintendo, as Nintendo typically tend to be, are behind the times stuck in their own little world. They've not gotten easier, but they've not gotten harder either. Their games are stuck at the same level of difficulty they were 15 years ago. Heck, for a lot of games they practically are the same game they were 15 years ago but with a single new mechanic you've mastered after an hour.
The biggest flaw in Nintendo difficulty though is that they try to be accessible to everybody, which often means catering to the lowest common denominator; the people who suck at games. This wouldn't be an issue, but one thing Nintendo fail to do is add a some sort of hard mode to 95% of their games. Heck, they've even started adding "impossible to fail" modes to games such as 3D World and Starfox Zero for the people who have absolutely no gaming ability, yet a majority of their games lack a hard mode...Or with something like Fire Emblem, they've started selling the hard mode separately for ?40.
There are some games that have kept their difficulty though (at least to me) like Donkey Kong games
Donkey Kong Country: Tropical Freeze is pretty much one of the only platformers as well as Nintendo games in recent years that I've found legitimately challenging at times. I have always seen the Donkey Kong Country series as Mario's hard mode though.
Fire Emblem Fates: Conquest
I've not played it yet so I have no opinion on it's difficulty but; one game out of however many games Nintendo have released in the past decade is an exception to the norm, not evidence that the average Nintendo game isn't astonishingly easy.