The reason is that in a monospaced font all the glyphs are supposed to the same advance value. This forces the ligatures to always take up the same space as two (or however many glyphs) are involved, which may stretch the ligature glyph in a non-intended way (if it is even possible to rasterise it that way).
Monospace fonts are also often used for programming (because they make sure that the columns line up consistently, regardless of the font being used). I personally don't see the point in showing ligature glyphs that do not correspond to the actual Unicode code points encoded to bytes in the source code.
Just my 2 cents.