As much as I dislike Microsoft, back in 2015 I used Windows Phone 8.1 for about 6 months and I absolutely loved it, the UI was so smooth and polished, even on low end phones, until WP10 came out and it ran like trash and I went back to LineageOS.
Internally they were two very different things. WP7 was their old windows mobile with a new skin, while WP8 was the actual OS they had been working on for a long time. They felt the same but were very different. I guess they didn't think it was worth the hassle trying to figure out how to handle that update.
I don't know what was the deal with 10 though (I forgot it even existed tbh).