The comoditization of embedded hardware designed happened over a decade ago. Have you heard of Kontron? PC104? Com Express? You seem to have missed the 2000's.... this is nothing new. These days it is amazing what is put on a DIMM module - far more than the Beagle Bone and Pi toys provide and at far lower unit prices.
The commoditization of these designs depended on several factors happening all at once.
First, processor power had to pass a threshold. Having a processor that is fast enough to handle an embedded system running a custom operating system (or more likely just a simple set of interrupt handlers and startup code) is a lot slower than the processor needed to run a full fledged kernel like modern Linux. The custom Software saves huge amounts of unit-cost, at the cost of time-to-market.
The second item that was needed was price point. Even $45 per unit is still high for the BBB black, but the RPi at $35 is pretty close. Even the BBB is close enough to work with.
Third, mainstream OS support. This is critical, because it turned a legion of higher-level programmers into embedded programmers. This, again, helps to reduce TTM
Last, the availability and maturity of simple to wire peripherals, and the availability of software libraries for using these peripherals. This is probably the most key part because you now have the ability to buy a modular set of components, and wire them all together with very little, if any, electronics knowledge and get a working system. Again, it all drives TTM, and in todays world TTM is everything. Just ask Microsoft how their tablet and phone business' are doing to find out how important TTM really is.