What people need to remember here is that accessibility software, regardless of platform, has generally arisen due to a couple of factors, with different emphases:
It is no surprise that Windows, the world's most widely used computing platform, has more than its share of screen readers because all of the above have been at play with the various players in the field at one point or another. I leave it to you, the reader, to decide which were bigger factors for each product.
Factor #1 is utterly lacking on iOS and Android. Whether one likes VoiceOver or TalkBack, both are fully mature products that do their job admirably. There can almost certainly be nothing as far as factors numbers one through three are concerned in these environments, and given the degree of maturity and broad acceptance of what is already available, factor number four has almost zero impact. Who's going to hail "the next VoiceOver/TalkBack"? Not many, that's for darned sure.
There are limited resources for implementing accessibility software, both monetary and otherwise. It makes absolutely no sense to try to open a market that's not really a market in the first place. Choices have to be made, and the probability of an entity that has the history of NVAccess doing a radical gear shift to other platforms makes very little sense from virtually any angle.
My dear departed Aunt Lila used to quote a rather salty old relative of hers when people brought up wishes such as this, that have all the odds stacked against them, and for good reason, "Wish in one hand, s*^t in the other, and see which gets full first." That sums up my thoughts about the feasibility and likelihood of NVDA leaping across computing ecosystems.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back