I bet the reason is practicality: try using full names ofdenominator/numerator in some involved numeric code. It's a mess. One may argue you need not access numerator and denominator explicitly that much but I think it happens.
I'm not sure if this fits with Phobos style, but they could be defined as "numerator" and "denominator" and aliased to "num" and "denom", respectively. That way, we get the best of both worlds.
