That is probably one major reason, but it's also important to keep in mind that starting with Lemmings 2, you can scroll vertically in some (many?) levels as well as horizontally. Just think about allocating a fixed area on the screen that could work just as well for a horizontal-level minimap as for a vertical-level minimap, and you can kinda see why it's bound to be a lost cause. Pretty much the only good system of minimap for that sort of generalized level sizes is something like what Clones has (you press a key and it momentarily displays minimap as an overlay over most of the screen).
Well, Cheapo has variable level size support, and 'solves' this issue by displaying only part of the level in the minimap, however more than is visible on the main screen. But it doesn't support scrolling with the minimap, which is pretty much the only use of the minimap in DOS lemmings for me, where border scrolling is slow for large distances. (Ok, sometimes, but very rarely, to see where a lemming is currently positioned in an overly long level. Yeah, just sticking to small level like ClamSpammer suggested is also a sensible solution
)
That said, Lix doesn't have the minimap, and I can do perfectly fine with that because, as Simon already wrote, it has support for an alternative way of scrolling: Right-Click the map and move the mouse, while the cursor will stay in place, the map will scroll into the direction you moved the mouse at a rate linear with the mouse movement, where the scroll coefficient can be customized (I emphasize this because the value I use is 8 times the default value).