Because to use email you need a dataplan. And a lot of users with pay-as-you-go sims don't have a dataplan. In fact, data on pay-as-you-go in the UK is prohibitively expensive. Remember, the phone companies charge a flat rate to send an MMS and nothing to recieve it. For email, they get to charge both sender and receiver.

On the other hand, it costs Granny nothing to recieve an MMS pic of the grandchildren. And should she ever get clever enough to want to send us a shot of herself living it up at the tea dance, it will come cheaply out of her pay-as-you-go credit without any problem.

Damn, now I miss my Granny.