There are many reasons:
1. Databases are designed for storing text. It's what they're for. They're not designed for storing image data, which is why we don't do that. I would not even hesitate to store a novel in a database.
2. Zenphoto is a photo and predominantly visual media software (including videos, which are very similar to photos you must admit). We simply think that if the majority of your content is in Text, then another program will be easier for you, rather than hacking a photo gallery software to display text files. That's what we programmers call 'inelegant' and 'a hack' and we mainly discourage this.
3. Helping you hack zenphoto is not our main priority - you're welcome to if you want, and we are helpful people, but we cannot spend time making this work for you, since you seem like you can't do it yourself, and we simply have very little time and have to focus on zen[b]photo[/b] features.
4. Honestly, we're trying to help you here. Zenphoto is a photo gallery software, if you want text you should use a text-based CMS, that's just going to be easier for you. We're not trying to say "don't do it" we're just trying to figure out an easier way! Because we know how hard this will be, and it's not something simple like you think it is.
If you still want to continue, you're on your own unless someone else on the forums wants to help. We simply can't spend time implementing every feature people want for them.
You'll have to do several things - you'll need a 'Text' class, or to modify the Image class, take care of reading the file from disk, and outputting its content. You'll also need to create database entries for each file you read, take care of sorting and ordering, figure out what thumbnail you want for text files to display in the theme, create new template functions to display the text, and more. Good luck.
Or, http://www.wordpress.com will already do all this for you, because it's already designed for that.