A few factors:
A) Emulation difficulty
B) Necessary specs
C) Open source, cross-platform emulator code availability
Dreamcast is more powerful than the systems you typically see emulated on consoles; more powerful consoles generally require more powerful machines to emulate, but there are some other factors. I don't know this for a fact, but I also assume it is more difficult to emulate than systems like SNES, GBA, or even N64. "More difficult" can refer to the fact that there is relative complexity to the system, as well as the architecture being fundamentally different from modern devices which require more emulation trickery. (The Dolphin team did a good job of explaining this and how Gamecube is equally or sometimes more taxing than Wii to emulate for this very reason.) Lastly, there are 5 or so Dreamcast emulators that have ever reached a decent state. Chankast, NullDC, and DEmul are all designed for Windows - they don't port well. Redream is closed source. The only viable choice that I see is Reicast, and compatibility is pretty spotty last I heard...
Aside from that, there is also the factor of user demand and personal interest. That is, console hacking is niche in itself, so to see a port requires of a particular console requires that someone with the right skills has a vested interest in the project. Dreamcast is beloved by many, but it's certainly not the most "popular" retro gaming system. Some individuals may prefer to port the more heavily requested and/or easier consoles to satisfy your everyday CFW user.
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