In respect to why Jupiter seems underrated, I don't know for sure. I think it might be our society. Though people are interested in financial success, I think the main modern interest is on sex, dating, pleasures and appearance. While reading the questions and answers on here, and looking through the most modern astrology books in the stores, I can conclude again that the main interests regarding astrology are those subjects.
Of course, it would be wrong to say that there's no emphasis at all in the expansion and success that can be had by using what is shown with Jupiter's aspects in a chart. It's just that many Jupiter aspects still require some studying and work on the part of the person involved. Once the dreaded words "hard work" are mentioned, I think most people return to those more engaging subjects of sex, dating, pleasures and appearance. I think some people have the idea that Jupiter is about winning the lottery, inheriting a fortune, or being lucky in getting the best job available without putting some effort into it.
When I was younger, Jupiter was a big focus for me in planning out my second career. I was already working in a specific career, but felt I was outgrowing my fascination with it. I had been into Sun signs, but I was slowly getting into full chart analysis. I was very interested in the idea of making a higher income, so I started putting focus on my Jupiter sign, placement and aspects to discover how I could best find success. Not everything has been a bed of roses, but I found what was said for those Jupiter aspects to be mostly accurate. I guess once I reached a place of satisfaction, I focused more on other things in my chart, and not as much on Jupiter.
You're inspiring me to focus more on Jupiter in the future. I'm in the process of finalizing yet another career involving Mercury. I was delighted to see your question standing out like a lighthouse beam.