Rationale behind freeware projects

Question

I've seen some freeware projects in the past where the author(s) invested a significant amount of their personal time and resources and never even considered charging for the software. A lot of these projects were donation based, and from what I've heard, donationware can never be a viable business model (even to simply support development costs) because most people choose not to donate if given an option. A lot of these projects eventually shut down because their authors could not sustain them further.

Granted, some people simply like making the community happy (or something), but if you're struggling to keep your project alive, why not charge some small amount such as $10 simply to stay operational? If people find your software useful (and a lot of people found those projects VERY useful) they won't have a problem paying such a small amount.

The question is: if you have a popular app that people like and download in great numbers, why not put a price tag on it? Why do it for free?