FaceOff on Hardware
Thresh's comments in BLACK
Kenn's comments in BLUE
Power Play: Hardware
I think it's pretty obvious that FPS games require a whole lot more computing power than your average RTS. This genre has always pushed the limits of technology, and nowadays it's virtually impossible to stay competitive without some serious hardware. This is probably the biggest drawback to FPS games - the need to stay ahead of the performance curve is crucial.
On the flipside, new technologies and products are constantly being developed for the FPS crowd (although not necessarily successful). Quake basically ushered in 3D acceleration, spawning an entirely new (and huge) market based on now-recognized names like 3dfx, Nvidia, and S3. The current push for 3D sound is also based heavily towards benefits in first-person games. The push towards more realism requires more and more PC firepower, and that demand will continue to push graphics processing and programming to the next level.
RTS games are generally simpler, and don't require the breakneck speeds afforded by today's high-end systems. While this isn't saying anything about how fun they may be, real-time strategy games are hardly industry-leading.
All right, back up a bit here. I'll be the first person to say that 3D action gaming has made an indelible mark on the PC (and console) industry, but you're really giving RTS games short shrift here. It's a great thing that we can all play a game of CNC2 or Starcraft on a P166, while Unreal would realistically require a powerful 3d accelerator and twice the MHz. But look at the overall picture. One of the biggest real-time games out there is Total Annihilation, and that game's got full 3D rendering.
Its sequel, TA: Kingdoms will have even more complex models, 16-bit source art, and a host of other features. Bungie's Myth series has been pushing 3D acceleration for some time now, and titles like Dark Reign 2 and BattleZone 2 are becoming just as graphically complex as today's first person games. One of the most-anticipated games of 1999 (if it's ever released, that is) is Command and Conquer 2: Tiberium Sun, which makes use of 3D terrain, dynamic lighting, voxel technology, and more. And of course, you can't forget the also highly anticipated Homeworld, just a precursor to the many truly 3D RTS next generation games.