Three reasons:

The hardware is significantly harder to get right. VR devices work well specifically because they don't care about your environment. AR devices, to be any good, need to have a semantic understanding of your environment. That's very hard to do, especially on the power and compute budgets that mobile devices allow. And an AR device that isn't mobile is a stupid AR device.

The software is significantly harder to get right. It's a lot easier to model a static scene and make some physics-based interactions in it than it is to try to figure out how to make overlays that react to a real environment.

But I think, much more importantly, the people producing most software in the immersive software space just don't care about your immediate environment. They care a lot more about giving you a canned experience. It's hard to find funding for anything that isn't some sort of media consumption. This is true across both VR and AR. In VR it's ok, because there is no environmental context to exploit anyway. But on AR devices, you just end up with a bad VR app: all the hardware limitations of a mobile AR device with none of the differentiating features. So because you don't get software that cares about your environment, you don't get good user experiences on the hardware.

Open up your iPad or HoloLens or Magic Leap app stores and take a survey of apps that are there. How many of them have any understanding of your environment? There are a lot that don't even take into account the "room mesh", the solid surfaces that the device can see--say nothing about what those solid surfaces represent! I'd estimate it's upwards of 50% on AR headsets and maybe 30% on iPad that do absolutely nothing with any surfaces beyond asking you to find a flat space on the floor. That's just crappy VR. As for the ones that attempt to understand what is in your room? Vanishingly few.

You can make pretty good consulting career out of making what is largely just a PowerPoint presentation in 3D: a collection of canned elements that the user can click buttons to get scene transitions and animations, all with a directed narrative that is trying to tell you something. Advertisers want it. Media companies want it. A lot of big-industry companies completely unrelated to media want it just to show off at conferences to "prove" they are "forward looking".

And you'll get a lot of those clients asking--even demanding--you make that as an AR app, especially on iPads. But it sucks. It's just not anything about what's good in immersive experiences. It fits a little better in VR. It still sucks in VR. But it comes around from backwards priorities. These companies start from wanting VR/AR and work backwards to a use-case. And often they lack any sort of experience or even actual interest in immersive design. What they want is to just do a marketing piece. There are very few companies that start with a use case and then find out whether or not VR or AR is the right solution.

But that's where the bread-and-butter money is. And it sucks the air out of the room. It leaves the real, good, immersive experience development to people who are independently wealthy enough to do it on their own, or to hobbyists hacking it together in their spare time.