First of all, the line about campaign settings splitting the fanbase killing 2e is not entirely accurate (not your fault - it's the way everyone talks about it - but there's more to it than that). The line has been around since Wizards purchased TSR in the late 90s (and is why they restricted the number of settings for 3e for a long time), wasn't actually just about campaign settings themselves but was more about "product lines" competing with each other, was based on a very different RPG publishing business model than what Wizards has adopted for 5e, and was really about killing the entire publisher (TSR) rather than "just" an edition.
When you dig down into the "multiple campaign settings fractured the player base and killed TSR" what it comes down to is that the publishing model they were using at the time depended on a monthly stream of income coming from new D&D books dropping every single month (for those too young to remember - yes, the 90s were crazy). But they were dropping so many titles each month that their titles were competing with each other - instead of having "D&D" supplements that everyone could buy it was like TSR was producing a half dozen different game lines where the players of each setting pretty much had to choose for budgetary reasons which settings they were going to support and which they were going to pass on. So TSR was spending money to produce a bunch of supplements each month but only getting a fraction of the playerbase to buy each of them, resulting in a spiral where they had to crank out more product to make the monthly budgets. It wasn't sustainable and eventually they went bankrupt (there were lots of other bad management choices that led to that - so it's not the sole cause).
When Wizards took them over, Ryan Dancey identified this as one of the things that was mismanged at TSR and they set up the new publishing model - still dependent on monthly sales, but fewer books each month at a higher price point and more that every single player would want to buy. The only setting books they published were the Realms, the new setting launched for 3e - Eberron, and the short-lived d20 Modern line which technically at least counts as a product line though I doubt it was really "competing" with the D&D books in any meaningful sense for player dollars.
That model carried over into 4e, though they started to dabble a bit more and released Dark Sun as a setting as well. Still the basic publishing model of a book a month continued but was no longer sustainable even in its altered form (in fact it's lack of sustainability is why we I think we got 3.5e and then 4e and then 4e Essentials as quickly as we did tbh). So they switched to a model of "a few books a year, mostly adventures, and run the brand with a skeleton crew so you don't have to make as much to pay for it".
That model means that they aren't creating separate product lines - they're creating a setting book that they release and that's it. There's no competition for dollars among the players because they have everyone perpetually starved for content to the point where most folks seem to be willing to pick up whatever book gets released with the D&D logo on it. Every release is "special" so instead of having Theros compete with 4 other products from TSR hitting the shelf that month it stands on its own, not even really competing with the Eberron setting released last year or the Ravnica setting released in 2018.
(Add to the fact that development costs for MtG settings have to be minimal - they mostly just have to develop game mechanics because the art and story pieces are already done. And MtG has a built-in player base that they want to get to crossover to become D&D players. So releasing a new setting from the MtG side of things every couple of years - with no added support beyond that - is probably worth the risk for them.)
