Because outside of the tech industry, not many people know about the .io TLD. If you have a company, I think it's natural that many people's first thought would be to go to <company name>.com (if they don't google it)
If someone saw that <company name>.com doesn't exist, they might think your company doesn't exist or is a scam
If someone saw that <company name>.com doesn't exist, they might think your company doesn't exist or is a scam