... largely as a consequence of several factors: interconnectedness, device development costs, functional flexibility, and winner-take-all market-capture dynamics.
Interconnectedness means that even if your secure, classified, compartmentalised, encrypted, logged information begins or exists on bespoke kit, there's extraodinarily good odds that it will transit or reside on other systems either on its way there or after being created. Networks are complex, with many components, and ensuring all kit is fully certified and cleared is all but impossible.
The costs of developing new devices is both falling (Moore's Law) and rising (hardware, firmware, driver, and software design is all rapidly rising in complexity). In virtually all cases, it is tremendously cheaper to begin with COTS (common off-the-shelf) hardware or software than to do a ground-up, greenfield, clean development. And where classified development exists, keeping up with improvements in consumer-grade kit is either impossible or remarkably expensive.
This claim rests on publicly available information, and it may well be that there are some exceptions. A few datapoints at least point to the likely costs. The Xen hypervisor has an advantage over vmware in that, by serving as a "shim" over the Linux kernel, it inherets the full class of Linux-supported devices. VMWare at least though the early 2010s was reliant on its own device development and featured a highly restricted HCL (hardware compatibility list).
The list of publicly-known top-500 supercomputers consists entirely of Linux-based systems. There is no public investment in either proprietary or bespoke supercomputer operating systems. Any classified work would have limited leverage from publicly-available development. Numerous of the publicly known supercomputers are engaged in highly-sensitive classified work. Evidence suggests limited, or exceptionally expensive, alternatives, if any.
Functional flexibility: what is still being called a "phone" is in fact a general purpose computer. And, for what it's worth, general-purpose surveillance-capitalism, state-surveillance, and ATP-surveillance platform, but I digress.
A slim glass-fronted slab provides voice comms, text comms, email, camera, calendar, notekeeping, Web access, geolocation, mapping, directions, e-book access, and a myrad of applications (though many of highly dubious utility). Both Android and iOS offer commandline access, though of varying completeness, reliability, and utility. (Termux now offers over 1,200 packages, Apple's iOS project remains fairly nascent.) This includes multiple development environments and potential well beyond the limitations of native Android and iOS platforms (already quite extensive).
A single "do everything" tool, that's sufficient for most of those tasks, will replace special-purpose bespoke tools in practice. It's an inevitable Desire Path (https://en.wikipedia.org/wiki/Desire_path). As a practical matter, workforce, corps, or agent discipline will be broken, and such devices will be used.
Winner-take-all market dynamics arise from several mechanisms, most especially positive-feedback network-effect loops of manufacture, development, sales and supply channels, developer ecosystems, and marketshare. Which means that not only will consumer-grade devices dominate, but a very small number of device or platform variants will dominate. Even highly-capitalised and capable firms experience frequent failure in attempting to dislodge established incumbents: Microsoft Mobile, in devices, Google+, in social networks, Intel's Itanium, in CPU design, Apple in cloud services, Amazon's Fire Phone. My point isn't that these are devices, but that they're the biggest players in the tech world taking on an entrenched contender and failing spectacularly.
Niche security projects are effectively playing in this field. They do not have to compete in the market with commercial offerings, but they do have to compete for talent, mindshare, tools, skillsets, and concepts. And they will all but certainly have to either interoperate with, or take into consideration the functions and features of consumer-grade kit.
Upshot: the worlds aren't separate, closed secure systems development competes poorly, and effective practice will blur all boundaries regardless.
There is no 'other' informatics ecosystem. We've got to make the one we've got healthy.