Great question. I started the project a few months ago and couldn't give much love to it so the idea was to have the MVP done earlier.
But basically I had 2 reasons.
1st reason is because I have a version that is already kinda working for the new versions. We're developing for both versions but prioritizing the Umbraco 13 first.
2nd reason is because even after Umbraco 17 is the LTS there will still be loads of wwebsites that will take time to move from 13 to 17 so the idea is to support both versions and all the LTS to come