Why can’t recipients respond to SMSes sent using an alphanumeric Sender ID?

A Sender ID is the name/number that appears on the recipients' phone, when an SMS is received. Among some supported countries, you can register and use an alphanumeric sender without additional costs. Unfortunately, a small select number of those supported countries only support transactional messages and require sender ID pre-registration. This process is performed only, if the sender ID is set to a mobile number, hence the replies go to that spoecific number.