Honestly, I think a large barrier here is there's no pre-existing platform yet. When developing an application for a computer with a regular screen, you have all sorts of supporting software (e.g. window managers, browsers, ui frameworks, etc). When developing for VR, you have to start over almost completely: you have to deal with controller input, different kinds of headsets, designing a UI.

For XR to take off, I think something akin to the metaverse needs to exist first, so applications can develop within its framework.