The main reason it is so hard to buy BTC relates to the issue of using US dollars electronically. Hardware wallets, aka wallets on a secure device, are the safest option; the drawback is that unlike popular apps and software, hardware wallets cost money. Bitcoin trading was eliminated and Japanese financial authorities started issuing cryptocurrency.
The reason that there are no stocks that can be bought through bitcoin is that, stock market is tied to the government which means that they are putting taxes on this stocks, dividends, and index funds. Bitcoin on the other hand do not possess this kind of ties with any government because of decentralization, though in the future this can be developed which I hope to be sooner because this is a big opportunity that can further spur bitcoin in mainstream financial circulation.