Ask HN: Why are so many .io domains popping up lately?

I count 5 stories in the top 120 on HN as of this writing, which are on github.io [6]-[8] or other .io domains [1]-[5]. Why are so many sites on .io domains? I'm particularly interested in comments from people who've chosen (or strongly considered) .io domains for personal or company websites -- why did you choose .io? [1] https://news.ycombinator.com/item?id=11085078 [2] https://news.ycombinator.com/item?id=11081711 [3] https://news.ycombinator.com/item?id=11082633 [4] https://news.ycombinator.com/item?id=11082359 [5] https://news.ycombinator.com/item?id=11077222 [6] https://news.ycombinator.com/item?id=11083520 [7] https://news.ycombinator.com/item?id=11082661 [8] https://news.ycombinator.com/item?id=11084580