That's very interesting to hear.
Search engines like Google look at so many ranking factors that it's hard to say that it was simply the Migration to Magento 2. There are, after all, migrations that leave sites with poorer rankings.
Overall, a migration is a chance to improve loading speeds, improve the usability of your site on mobile devices, start fresh with cleaner coding, and more. These are all the sorts of things that can give you a leg up on your rankings.
Without a test of your old site while it was live, and now your new site, I don't think it's possible to definitively say which ranking factors your site definitively improved for, but since your site rankings moved in the direction you were hoping, luckily, you don't really need a post-mortem.
I'm glad to hear that this worked out for you! From my experience, while it may not be completely because of modernization in Magento 2, when sites start fresh (while keeping some elements, like URLs intact), their SEO often improves - but this also has to do with things that may have been holding them back in their old website iterations.
Best of luck!
