What are the recent innovations that led to the uptick in popularity of VR headsets?
Innovations include:
- Decreased cost. While the original high end HTC Vive headset sold for $799, there is now a version with improved specs (Oculus Rift S) which costs just under $400 at time of writing this answer (02/12/2023). That's still expensive by current standards but not prohibitively so when you compare it against even early adopter prices such as those of an Oculus Rift Development Kit 1 costing about $300 USD then suddenly the difference in price between development kits and consumer ready hardware is no longer so huge compared to the savings on development expenses that comes from being able to try your own hand product designs rather than relying entirely upon what third party vendors will make for you based off templates supplied to them. 

There has been a large amount of research into various techniques designed specifically towards improving motion tracking and other areas of quality control especially relevant since most VR applications today are content centric.
One can look up academic papers related to "High Resolution Eye Tracking in Virtual Reality Head Mounted Displays" or some variant thereof for more details on where all this is headed however suffice it say these methods allow far greater fidelity and precision than before without the need to use markers on skin to track eyeballs - and that may have consequences well beyond the obvious one of better graphics! 
Another major area is ergonomics making devices usable over long periods while reducing eye strain, discomfort, nausea etc... For example many companies like SteelSeries, Xai and Logitech produce gloves compatible with VR systems which do not require buttons on their fingertips. The latest trend I am aware of would be integrating haptic feedback directly onto hands via vibrating material sewn onto finger tips instead. It's likely some of the aforementioned technological advances are contributing here also