Xbox One:

1. Cheap prices and great deals (past the initial Kinect blunder).

2. Momentum from X360.

3. Decent "true" exclusives for the first half.

4. It took a while before everyone knew exclusivity are a thing of the past.

5. Steam yet to explode in popularity.

6. Sony's first party titles only becoming so popular in later years.

Series X/S:

1. Two years of brutal PS5 shortages.

2. GamePass garnering attention during COVID.

3. Solid hardware.

Beaten to it, but like 85 million people had a 360, and brand loyalty can be a powerful thing, so a lot of them jumped to Xbox One in spite of its shortfalls.

Over time though, as the platform continued to bleed, more and more people decided "fuck it" and jumped ship, leaving just the hardcore adherents.

Xbox would probably have had a much harder time if Sony hadn't initially done so poorly with the PS3. The success of the 360 probably helped the next gen as well, and the success of that gen probably helped this one too. I guess it also helps that Microsoft did some decent moves back in the way. Along the way, they started losing more and more of those moves.

I guess there's more to the story, but I think that's probably much of it already.

What got me invested in Xbox back in the ancient and distant era of 2001 was... Exclusives.

Even though I was a PC gamer and got Halo and Fable... Most of my friends were poor dirty console peasants, so they bought the Xbox... So I had to buy an Xbox.
And we would all lug our chunky back-breaking CRT's and beefy consoles to school and the teachers would lock us in the Library for the entire weekend (We were living off Pizza and soft drinks) and we would game with Halo over the local area network (That I ironically built when I was in school) all night long.

Then Halo 2 happened.

... Then the Xbox 360 happened.
And ironically the reason I bought the Xbox 360 was because of Perfect Dark, one of my favorite games on Nintendo 64... But also because of other promising exclusives like Halo 3 and Fable 2... Which were coming off the hot backs of Halo 2 and Fable 1 TLC. - Then Gears of War Happened.

Microsoft was offering us PC snobs the games we liked on console... And that is arguably why I probably own every console today.

I still invested in the Xbox One... Again for the promise of exclusives (Mostly interested in Halo and Fable on Xbox), but they were fizzlers, Halo 5 was garbage and the Master Chief Collection was an unplayable, broken, buggy mess.
But... Microsoft did offer compelling backwards compatibility which allowed me to play the games I used to love... But better.

Then the Xbox Series happened. And whilst I was a day 1 owner, I was hoping for a return to form as the Xbox Series X hardware is truly fantastic, it just needed those games... But this isn't strictly a Microsoft issue either, I was a Day 1 owner of the PS5 and there still hasn't been any compelling exclusives that really have made me go "wow". - And majority of games release on PC anyway.

Halo Infinite did drop and it was definitely built with the Xbox One hardware in mind, it was barebones, it was boring... It wasn't the height that Halo used to be known for.
The Perfect Dark reboot imploded, Fable still hasn't arrived and many of the promises of amazing games just never eventuated.

*****

With the advent and domination of digital gaming on consoles and with everything coming to PC, this generation is likely to be my last console generation... Be it Playstation or Xbox, there just isn't any point to owning those platforms anymore.
Plus if I am going digital, I am sticking with PC where I can play those same games and play those games everywhere on any PC hardware I want... Whether it's handheld or a TV. One library.

Truly makes me think how Xbox One would've sold in the end if it never backpedaled the controversial policies. I'm thinking more than the original Xbox, but less than Xbox Series X/S. Might've been enough to kill Xbox hardware if Microsoft chose it. Or Xbox Series could've been a comeback. 

Xbox One really wasn't a bad system in a vacuum, heck as per a recent thread I made here even the Series can be said to be a good console in and of itself.

The problem is that the competition was just better, and once Xbox exclusives started coming out on other platform there just wasn't much reason to buy an Xbox any more.

There's also just the fact that Microsoft has more money than god and can just plough on and on while eating losses that would make pretty much any other console maker call it quits.