Actually, the real reason was that Doom ran in "Mode X", where it was actually faster to rasterize vertical strips rather than horizontal ones. This worked out especially well in Wolf3D and Doom, since perspective correction in the vertical direction was never needed.
Actually it didn't it ran in the bog standard 320x200 256 color mode. mostly because using "mode x" back then wasn't as widely supported on lots of low end machines.
Looking at the Doom source, it seems that the actual 3D game view was rendered into a linear buffer, and subsequently blitted onto the mode X surface. Some UI elements went straight to the screen.
(The DOS Doom source was never publicly released, just the Linux port, but there are remnants of some DOS code in the archive. I'm specifically looking at R_DrawSpan in README.asm and V_DrawPatchDirect in v_video.c)
