My guess Valve didn't want to simply make Half-Life 3 for the sake of it. Half-life titles were always revolutionary to FPS genre both in game design, storytelling and from the technical perspective by introducing the Source physics engine. I think this is the reason why HL: Alyx is going to be a VR exclusive. They don't want to simply make a game. They want to make a game-changer. Anyway, this is the right time to go back and revisit the previous games in the series. And I am one of the few hopeful ones who think that the release of Alyx will mean the return of Half-Life franchise.
I have no interest in this game nor VR, just getting that out there, but I do have my own thoughts on this.
My laptop bought just this month could (on paper) run this, as could my 3yo self built PC if I ever bother to repair it, so it's not as hard a game to run as people say it is. HL was always gonna be PC exclusive because Valve (even though the older games were not, but glancing past that), so combine that and the age of HL2 means the vast majority of people who are serious about this game and this franchise will probably have an adequate gaming PC of some kind by now anyway. The VR headset is the killer. $400 just to access the limited range of software it's made for is a big ask.
A big source of the negativity for this comes from the very vocal group of fans who want HL3. Let me make my view clear, HALF LIFE 3 IS NEVER GONNA HAPPEN, and for good reasons. The concept is dated due to the previous game's age making it all but impossible to do something that stands out the in what's become a very saturated market for FPS games. Most modern FPS games basically are HL2 and mimic the design, so it's hard to make a new HL game stand out. That added to the fact the diverse and high expectations from fans with regard to the story, and it would be impossible to make HL3 that's true to the game's core fundamentals, yet still distinguishable as an FPS of today.
The only way Valve could actually make a new HL game is by doing, well, exactly what they have done. Innovate, try something different, but most importantly, NOT make it Half Life 3 as such a break from tradition would just create a huge backlash. Yes, this was always gonna piss some people off, but at worst this was the lesser evil, and I admire them for having the courage and wisdom to follow this through. FPS games have massive potential and ease of compatibility for VR, and based of what we saw in the trailer, this game looks particularly well-made to work with the interface. This is a game being developed by people who know the platform and who know with they're doing. I think it looks brilliant, frankly, even though I almost certainly won't buy it. To summarize, the vast majority of HL fans who won't by this game won't buy it because it's VR, and that's fair, but I think that complaining about it because it's VR is a tad petty and no reason to get mad at Valve. It was either this or some other radical way to come out with a new HL game because HL3, despite what I'm sure would have been Valve's bet efforts and intentions, would have let people down in the end.