Because today only in the sphere of altcoins you can earn, because bitcoin has already a very large capitalization and in bitcoin large amounts of money are poured in, which today most people of this planet do not hesitate to pour in.
Altcoin is the other way on how you could earn money through investing on Trading if you don't have enough money to buy bitcoin as an investment. We have different altcoins in the market, they have all different prices and potential, therefore, before buying some altcoins, you must know all of their information if they are good to trade in the long run, buying random altcoins is too risky.
And also before you invest on altcoins, you must know what you are capable on doing Trading because it is not easy like what you are thinking. It requires time and effort on observing the market.