The difference between the keyed bugle family and the saxophones is that with brass instruments you're generally playing up in the second order and above - brass instruments rarely use the fundamental, they call it "pedal tones" but it's not used much in music. So the tubing has to be much longer for the ordinary range. And as a result the brass instruments are generally made up of a roughly cylindrical section and then a roughly conical section. Now if you take something like that and interrupt it with toneholes on the side, first of all you're losing much of the effect of the bell - which is greater on brass than on a saxophone. And then you're also changing with each tone hole that's opened the ratio of cone to straight in the tubing.
By contrast, whichever tonehole of the saxophone you open, the general proportions of the bore remain constant. This is why the saxophone registers are quite similar in tone compared to - for example - the differences between chamuleau and throat tones on clarinet.
In the end, it's turned out to be the best compromise with brasses to have the length-changing occur in the more cylindrical portion of the bore, with valves, and use the same bell for all notes. Even though valved brass also have a variation in cone-to-straight as the valves are used, the tonal and acoustical effects are less when you make those changes down in the cylindrical section than when you cut off portions of the bell.
In truth, the acoustical compromises of the saxophone are among the smallest of all the instruments. It's one of the acousticallly most perfect of the wind instruments.