The main reason is because they want to sell more .info domains at cheap prices so that you pay the full price when you go to renew the domain the next year.
Because ".com" is the most recognizable and memorable extension, and a good one is easier to brand. It is also rare to find one that makes any sense, therefore there is more demand for them. Nobody lowers the price when the demand is high.
The actual prices of the TLD's (com, net, org, info, biz etc.) are $9.99. The $1.99 .info domain or $7.49 .com domain is actually a promotion offer which is valid for a limited period only. When you renew the domains, they will renew at $9.99 unless there is a renewal promotion like $6.99 .info domain.