To take a guess, this might be the reason indeed.
Yeah, I put myself in the shoes of a budget IEM manufacturer. If I want to use a BA, I’ll just order a ‘traditional’ BA, which is a much more compact package (not to mention easier to tune with standard tubes and dampers).
But let’s say I got a really good deal on a boatload of empty shells that required a 6mm dynamic driver. Well, a 6mm dynamic driver (of the budget range the SR is in) literally costs a small fraction of the SR drivers. And in my hypothetical ‘good deal on a boatload of dynamic shell’ scenario, if I really wanted to I could manufacture a simple silicone holder with a 6mm round OD that fits a standard rectangular BA. In fact, we already see this type of BA holder on many IEMs (although it’s purpose isn’t to fit in the exact space of a 6mm dynamic driver, but you get the point).
Also, BA drivers are more susceptible to corrosion caused by sweat, as well as being intolerant of physical shocks compared to dynamics.
I wasn’t really blown away when I heard the SR. It honestly sounds like an average dynamic, but with less sub bass. Since I’ve heard dynamics that were much, much better, I guess I the million dollar question I’d ask myself is ‘why choose the 6mm SR over a good 6mm dynamic’?
I’m sure there’s some reason or usage case when the SR would be superior; I just can’t think of it off the top of my head. And my guess is that other IEM manufacturers asked themselves the same question and came up with a similar conclusion as me (hence why we don’t see widespread use of this oddity, nor why Knowles never expanded the product line to include other diameters).
It was still neat to experiment with though, and I’m glad I played around with a pair to check out the tech.