It's a revenue generation for the SP. They get paid when they receive a text. It's probably good for the girl, as it's anonymous for them, deters time wasters and makes a bit of pocket money.
Not sure why you like it. You can actually just text a girl directly without an AW interface or send an email without paying for a text.
As for girls not replying? Well, if they only have a private gallery and a aw sms chat function then they are just out for cash.
If (for some reason) you are really asking why people are not using the aw sms text function, it would be because people see it as a scam. That's if you're one of the guys that has fake profiles on aw to generate credits. Not saying you are, but you'll admit it's an odd question to ask here.