Bitcoin enjoys actual demand, and that on so many different levels/aspects, where altcoins for the major part are only functioning as gambling tool for speculators chasing quick gains. But if we look at how the majority of the top altcoins have been doing, then there aren't many of them lacking growth -- crypto in its entirety is booming. People who look at Bitcoin as being too expensive to buy, even when they understand how divisible Bitcoin is, try their luck in the hope to experience a similar booming effect in the altcoin market. On top of that, altcoins are an easy tool to switch to when people look to hedge Bitcoin.