..because a lot of design work has gone into them, and they have more stuff that makes them more usable.
Off the top of my head, I think this device would be slow to use, and would require a fair amount of skill/education (you have to be able to spell words in morse code - fine for many, but a problem if you've got broad developmental issues). Physically, for many users, taking 15 breaths to spell a word is going to be slow and (for some) very tiring. As you get tired and aren't in perfect control of your breathing, even small errors in input would cascade down and the resulting words would be unintelligible. Because Morse code isn't actually well suited to the task.
Again, off the top of my head, it seems like it'd be much faster to choose common words (or pictures) off a screen (a screen wouldn't add much to the cost - probably only a few dollars). Then you could use continuous breaths - perhaps with the device measuring breath velocity when the user is able to moderate that - to alternatively move a cursor x+y (and then a short breath to "click") to choose options off a grid. With some training, I bet you could get most common words in 3 breaths this way. Or, when more choices are required, you could use the same mechanism to press keys on a keyboard (though, again, you'd definitely want word completion). For heavy users without developmental issues and with good breath control, you could build out a shorthand type system that might be fast enough for reasonably paced conversation (using breath length/intensity, and lengths of pauses between).
I'm not saying this isn't a cool thing, and it could certainly be made cheaply (though, again, a very cheap screen would make this a lot easier to use). But it's pretty much the "hello world" of assistive devices. I like when people make new things, and I like effort/attention going to problems like this, but I'm tired of how these articles tend to belittle the work done by others who've approached the same problem. It's not that nobody ever thought of making a simple breath control system before. Most likely everyone who approached the problem started by making a device much like this to test the initial breath control... and then they made fancier ones that worked better, based on the feedback they got from real users. Pretty much any suitable engineer, when presented with this problem and a cost constraint, would be able to make a similar or more usable device.
That's no disrespect to him (he's doing a good thing) - it's disrespect to this kind of breathless reporting. No, some teenager in Britain didn't come up with a way to triple the speed of your internet connection. No, some science fair kid didn't come up with a way to make solar panels 100% more efficient. People seem to love the base story here (teenager shows us all the way), but the stories almost always well overstate their case.