Mobile users don't have MMS enabled phones, or even if they have 'em, brands and media owners don't know how MMS look like on their phones. This is the problem slowing the development of MMS marketing campaigns. On New Media Age, Pamir Gelenbe of Flytxt says that they have a very limited control over the quality of the final MMS.
Basically the problem is that users' positive experience cannot be granted. There is the risk to spoil consumers' attention towards MMS campaigns with low quality messages.