One of the reasons I like OpenBSD is the developers are very forthright about why things can't or won't work. Reading the misc@ mail list is a great way to learn about the issues they face trying to get documentation. There are non-trivial issues with both acpi and efi. The developers reverse engineer what they can.
Instead of asking "Why doesn't OpenBSD have better support for $hardware?" we should be asking "Why don't vendors post more public information about their hardware?"
Anyone who grew up in the 70s and 80s buying electronics probably has very distinct memories of getting schematics and diagrams with their new products (or could order them cheaply). My first cw-band radio came with a full electrical schematic. Now, it's a crap shoot. Some of the blame lies with the industry as a whole. Much lies with the USPTO, or more precisely, the laws governing patentability and duration of patents.
The industry is to blame because it's easier to not to. Even if a retail vendor wanted to release good doc sub-component vendors may refuse to allow them. Why? In part to protect themselves from copycats. In part to protect themselves from patent lawsuits.
Patents are the another aspect of the not-so-secret problem. They're all violating somebody's patent on something (at least in the eyes of the patent holder). Whether it's in the fabrication process, a "method" of calculating or who knows what, someone has a claim. The more a company expose about the inner workings of their devices the more information patent trolls and competitors have for pursuing license (revenue), agreements. The smartphone patent war we're seeing played out in the courts is one example of the problem.
Yet another aspect of the problem is self-serving vendor "standards". EFI began as an Intel initiative. Intel later handed control of the spec over to the UEFI Forum, a non-profit corporation. The goal of EFI isn't so much to fix BIOS as to further vendor interests, whether to protect their "IP" or lock customers into using their devices in vendor "approved" ways.
Contrast that to Open Firmware (OpenBoot) which began as a Sun initiative and later became an IEEE standard. Or LinuxBios (now coreboot) which is an open source replacement replacement for both BIOS and EFI. Coreboot has made some progress but it requires vendor participation to make critical details available for implementation. You can guess how well that's going.
If the OpenBSD project were willing to sign NDAs and/or accept binary blobs there would be better support of technologies like suspend/sleep. But they're not willing to do so. Rather they work with vendors who are willing to share details, reverse engineer where possible and do without when neither option is available.