I’m currently browsing through a plethora of catchword fonts1, trying to find a good fit for the title on a book cover.
Most of these catchword fonts contain many of the same words—some only in English, some also in other languages—but as far as I can tell so far, there is one thing that they all, with only one exception so far, have in common: no indefinite articles.
All of them contain the definite article the (as well as le/la/les if they include French, der/die/das/dem/den if they include German, etc.), but only a single one contains a or an (or its equivalent in other languages).
Though there’s much overlap, there’s also a fair bit of variation in which words are included, so it seems like more than just pure coincidence that this particular word—the fifth- or sixth-most common word in the language—should be completely absent from pretty much all catchword fonts. In the type of situation where you’d want to use a stylised catchword to write the or of, it seems highly probably that you would want to do the same with a(n).
Is there some specific reason that I just cannot grasp for omitting a(n) in particular from catchword fonts?
1 Fonts which have special, predesigned glyphs that represent common short words like ‘the’, ‘for’, ‘of’, etc., usually with some kind of visual border or similar effects, especially made for use in logos, titles, and such things.
There is no definitive answer to your question, but one line of reasoning on behalf of the type designer is that catchwords usually shorten a string of text. When a “and” is designed to sit within two arbitrary words it allows for making a shorter headline or word mark or catch phrase. A reason short words like “a” or “an” being absent could be the perception that they do not shorten but instead bloat up the text and thus their use and designing for that use does not come intuitively.
To further underline this reasoning look at how many fonts have a Numero or Female and Masculine ordinals, all of which shorten the text and are abbreviations found in handwritten language.