Because of marketing and mind share, easily two of the biggest problems Microsoft had this gen. They had to make a console which is better than anything Sony can offer and they did. They had the image to sell a console with worse specs for a higher price and wanted to recover from that and they did exactly that. It would have been a farce to release a 3tf console because who wanted to even buy that?
At $499 releasing in 2017 Xbox One X could not gain a big enough install base before the shadow of the next gen consoles started to cast. Even in this thread people are blaming the lower install base for the lack of optimizing in the games where Xbox One X isn't showing it's dominance.