Hi M M, 

The simple answer to your question is that it is just a matter of priorities. Fonts didn't make the cut yet. 

Why? Because Indigo is first an interaction design tool, and while typography certainly has an effect, especially on the emotive axis, it is less crucial than, say, being able to design interactions themselves. In fact, there is a whole host of things arguably more important to interaction design than custom typography (and styling more generally). Stylistic things that do not affect, for example, information hierarchy, are lower in priority for Indigo than those that do. Indigo is not meant to be a replacement for visual/graphic design tools.

That said, we did spend time to ensure that we included a clean, sans serif font as the default (Open Sans) as well as a number of serif, sans serif, fixed, and variable width fonts. These should normally be sufficient to effectively explore most interaction design, and yet, we are not saying we will never have custom font support nor more styling support. It is simply a matter of priorities.

Given this, I suggest you go vote on this idea to express your support for it. The more people demand it, the sooner it will get into Indigo. Feel free to browse the other ideas there, add your own, etc.  

Thanks for the feedback.