I mean, Learn actually saw a bit of play in Standard. It's just that it was pretty weak power-level wise (so it didn't break into eternal formats), can't be used at all in Commander (because it has a "no wishes" rule), and fits kinda awkwardly into Cube. I also think it's a little unfair to lump it in with a bunch of other mechanics that add overhead to the game, because it's not like wishes are a new concept in Magic.
That's fair. I just never liked them from the jump and always found them to be clunky in implementation. Wishes being more open-ended never really bothered me (to be fair, it's also before my time), but tying in this mechanic to lesson cards as the only target(s) made it far less appealing to me. At least wishes can actually get good cards ala Karn wishboard, but lessons are just so unimpressive and weak that I never had any interest in messing with them from the jump. Strixhaven was just a big miss all around for me so I definitely wasn't the target audience for this.
I feel like a lot of this comes down to the need to still needing to come up with a 2-3 new mechanics for every set (like they've always done) without the block structure slowing down the accumulation of new mechanics. Which... can I complain about the rate we get new mechanics for a moment?
The two Standards Scars of Mirrodin block was part of had fifteen and thirteen new mechanics respectively, with each block contributing six-ish new mechanics (Zendikar had some extras because I'm being generous and calling quest enchantments/multikicker/traps "new mechanics"), with both standards collectively having twenty-two new mechanics to learn, with the big ones getting 2-3 sets to develop.
Current Standard has something like thirty novel mechanics, depending on how you count them. If we stretch a little and count returning mechanics, the two older Standards get two extra mechanics each (kicker + imprint and imprint + flashback), while current Standard balloons up to forty-ish. That's a lot of complexity to keep up with.
The reason why older blocks accumulated new mechanics more slowly is that they were essentially just one big "main" set with a couple of smaller expansion packs that extrapolated on the stuff it introduced (both mechanically and lore-wise), with there only being a handful of exceptions (Innistrad, Lorwyn/Shadowmoor, Ravnica, Tarkir, and Zendikar — yes, Scars just so happened to be flanked by two of the five "bigger" sets). If you had a major new (or returning) ability or keyword (or frame! Innistrad added double-sided cards to the game), it'd be in roughly half of the sets in Standard.
Contrast that with the current design regime, where non-evergreen/deciduous stuff generally shows up in one set, with there being a couple of exceptions (the two Innistrad sets were an unofficial two-set block and had some extra overlap, while Powerstones were hinted at in Dominaria United and focused on in Brother's War). It honestly almost feels like the design team is consciously choosing to not repeat any returning mechanics until a set that used it rotates. Like, am I the only one who thought it was a little too neat and tidy that we got kicker in Zendikar Rising and then got it again in the first set after ZKR rotated?
Yeah, this is definitely a problem in that new mechanics just don't have the depth necessary to be worth exploration most of the time.
Variants of kicker or things that deal with +1/+1 counters like Backup are easy enough to integrate with pre-existing cards, but there are a hell of a lot of cards nowadays that are printed to mention one off mechanics that make them awkward for inclusion outside of a dedicated Limited or Standard environment. There's just not enough room for these mechanics to breathe and really make them feel special. Like Streets of New Capenna was from a year back and I can't recall the majority of mechanics in that set off the top of my head despite having 3-4 cards from the set in my cube.