I think it's because forex and the stock market are way more widely known to people and crypto is quite more niche for trading than them. It's easier for gurus to sell their forex trading courses compared to crypto.
Also, forex and the stock market are way more stable and are way better for day trading compared to crypto. The low volume of crypto causes huge pumps and dumps in a short period that can easily liquid your position. So it's more about the experience and knowing how the market will react at the moment rather than knowing technical analysis, patterns, and trend lines.
If you've learned technical analysis in any market, doesn't matter which one. If you decide to move to another market, you already know 60%-80% of the things you need to know to trade in the new market. The only things that change, might be the numbers you must use for some of the indicators such as RSI or MAs. Other than that, it's all about learning how the new market reacts to strategies and trend lines, which, someone can explain in a course, but it's not impossible for you to learn it by analyzing the chart and learning the new market.