The same reason you can't send a text message over WiFi (with the obvious exceptions of free texting apps and such), because its not using a cell tower. It would be awesome if you could send MMS over WiFi, but I doubt carriers like Verizon will start doing it.
I guess what I am getting at is that there is no actual limitation. The technology does not have a limitation preventing sms mms or anything else from being sent over it. Think about this for a moment. Without using anything special I can hold a text conversation with people on phones from my pc or laptop. Neither of those are on a cell tower and yet the phone can communicate with them. Your phone can hit your home router which means it can easily communicate with another device over the wifi on that router. So why can't it send over wifi again?
Either because your specific model or carrier does not allow it for whatever reason. That would be the only limit. The technology contains no limit of it's own by default.
Not that I am trying to argue or anything like that but I just don't see the reason it would be impossible to do unless there is a carrier lock or other limit put in place beyond the available tech. Something put in place afterwards to prevent it specifically.
If that is true then it also means it can be defeated and wifi can be used for text and other stuff kind of why I want to find out what the exact reason for this is.
From everything I am reading though it should just work. If it doesn't then there is a problem or the carrier/manufacturer has prevented it beforehand.
Either way you should be able to find a way to do this either by fixing the problem or removing the locks.
UPDATE: From another forum where we are talking specifically about samsung:
Has anyone had issues sending/receiving MMS over wi-fi on the S III? I regularly use Handcent and contacted them about the issue, but there hasn't been a solution. I've tried both the stock messaging system and Go SMS with the same results. I noticed that this was also being discussed over on XDA Developers, so I thought I would contact Samsung via LiveChat today. I was basically told that Samsung designed the phone
not to send MMS over wi-fi and only over the carrier's network. Does this seem correct? This is the first Samsung phone I've owned, so I don't know if this is something they've done in the past or if this is a new thing for them.
Here's part of my transcript...
"I have run a check on the database for you and I am sorry to inform you that, the phone is designed in such a way that there is no option to send MMS messages using WiFi network as it can be done only through AT&T network."
"The Samsung phone is only designed in such a way that there is no option to send MMS messages using WiFi network as it can be done only through carrier network."
Are other users not experiencing this, or is everyone else sending their MMS over their network?
Sure this is an S3 we are talking about but it is samsung and the S3 is way better than the centura if they left it out of the S3 then the phone we have sure as hell didn't get it either.
Like I was saying the technology by default contains no limitation against sending it the hardware manufacturer in this case samsung has simply decided to leave that out of it's hardware apparently because the carrier someone other than AT&T (I'm guessing verizon) did not support it either. It sounds like AT&T supports this and there should be no issues on that network or the phones if you wanted to send over wifi instead of 3g or 4g