Initially, names were easier to find under .io than under .com. I don't know whether this is still the case, and it is by itself not sufficient reason--- there are now tons of TLDs available, but hardly any see any use. But during the boom of the 1990's, tons of .com names were claimed, while .io names started being registered only in 1998, and didn't take off until later. .io names are likely to be cheaper to acquire, particularly for common words.

For example, "context.com" initially was registered in 1993, but context.io was still available for somebody to register in 2011.

Second, .io is like .ly or .st lending itself to "puns" like techfol.io or card.io or easily-pronouceable or short names like context.io. I/O is also an existing computer term, so is recognizably "tech-y".

Finally, company names have fads just like everything else. Particular letters or patterns become popular because people want to sound "modern" or "cool", or they avoid naming patterns that sound dated. "dot-com" sounds a little bit like the 90's.

But, ".com" is still overwhelmingly popular. FundersClub found that 87% of startups that raised Series A founding in 2017 used a .com name. But Y Combinator startups in the last class were down to 68% "dot-com"s. Source: https://www.forbes.com/sites/christophersteiner/2017/05/03/most-startups-still-believe-they-need-a-dot-com-domain-but-that-is-changing/ (A note at the end suggests that startups which successfully raise money may use some of it to buy the .com version of their name!)