It's simply way cheaper than alternatives. Index is 999$, Quest 2 is 300$. WMR, Vive and Rift S are not sold anymore and there is basically no one else left on market.
Price is a factor, agreed, but the I think the right way to frame the Quest 2's success is value. USD$300.00 for what the Quest 2 offers is incredible value. It's not just a cheap piece of kit, it's also stupidly
good for its price tag. You don't need a PC, which already gives it a huge value addition to the average consumer. On top of that, if you do have a PC, you get a top-tier VR Headset to play the highest quality games on - hence its position in Steam's hardware survey. And on top of that, if you have a WiFi 6 router, you can have it all wirelessly. Half-Life: Alyx without wires? That's a borderline religious experience. As for other headsets, there are actually a good number of alternative VR Headsets on the market, each pushing a particular niche. There's several different versions of the Vive available, for example. But, none of them are offering the same value as the Quest 2. AUD$600.00 for the Quest 2, or,
AUD$2000.00 for the Vive - which
also requires an expensive gaming PC? It's not even a competition at that point.