My reasons are:
1. Bulma is based on Flexbox. Buefy is just a framework that adds Vue.js flavor on top of Bulma. .. quite #cool, I must say. Both are very well documented and looks quite good. Nuxt CLI let you choose Bulma or Buefy, but I prefer to use Buefy directly because Bulma offers only a CSS.
2. Page loading speed. After building a PWA starter with Nuxt and Buefy and deploy it on my server, .. the first instinct was to test the speed with Google Lighthouse tool. I was using two deployments, PWA and SSR. The results are quite identical and #impressive at the same time.
PWA / SSR - using Nuxt / Express and Buefy starter (sources here):
After this 10-min R&D experiment, I've decided to start using Buefy in production and build a tool that converts automatically Bootstrap themes to Bulma / Buefy.
Thank you for reading!