Easy, HDR10 was available from the get go. And as such, everything ends up supporting it - all the TVs, receivers and sources support it.
Dolby Vision was available a couple of years later, and it took longer for devices to start supporting it. Plus, Dolby wants royalties for its use, so hardware manufacturers are much more reluctant to license it and make their product more expensive. It's why there are other standards like HLG and HDR10+ - cheaper licensing or such.
Dolby vision for streaming services is different from Dolby vision on discs. The one on discs requires special licensing, which is why no media player supports playback of Dolby vision disc images. Some rippers though offer the ability to convert the disc version to the streaming version, which lets the ripped video play back in Dolby vision.
As for why streaming services support it, well, I can only think Dolby is trying to encourage it's use to get the licensing money. And it seems that it's easy to make existing hardware support it which is why devices that can't do HDR can support Dolby vision,