Which Characters? And why Italic?

Why do certain sets of character get included and not others? For a variety of reasons: some have only the originals to go on and it would be pointless and disingenuous to continue, others are constrained by the size of the character boundary, a few simply become unwieldy and unseemly in a certain style (Roman numerals in Fraktur? Definitely a contradiction in terms). Ultimately there can be no hard fast rule.
But, if you find there is a character that has been left out you can not live with out feel free to contact the foundry. We will try to implement if feasible. For example if the Lower Proto-Swabian tailed umlauted "zed" has been omitted and that might be useful for transcribing the sole known example recorded in the burial index of "Keinberg" that could be accomplished. But the foundry would prefer to keep things attached properly to the Unicode standard. But it acknowledges even that at times is not comprehensive, though they are making an awe inspiring effort.
One last thing. Why make an Italic character set? Well, Aldus Manutius back in the 1500's decided that he should model a typeface on the popular script of the Humanists of the day. It became wildly popular. But more importantly to us, it has very subtle and not so subtle differences in letter structure. The most obvious is the leaning that most word processing programs can automatically apply to any font, but the real differeces are in the way the letters are formed with a more cursive aspect which can not be achieved by the psuedo-italic of the auto lean, which is really just an Oblique version of the Roman face. Examine the following:
Q, a, e, f, g, p, q, r, v, w
Q, a, e, f, g, p, q, r, v, w