What is the psychology of people who don't reply to text messages? Is it rude or too nervous?

I think besides your two possibilities, there are the following possibilities. The first possibility is that the other party is too busy to reply to text messages. The second possibility is that you are not the person the other person cares about, and the other person doesn't think it is necessary to return. The third possibility is that the other party doesn't like answering text messages and hates writing letters. The fourth possibility is that the other party may feel that the text message is not formal enough to insult the other party. The above is my analysis, which may be inaccurate or incomplete, but I hope it can help you. Of course, if you feel good, you can also use it.