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Well imagine being an investor then investing to Bitcoin first. You would think of diversifying your investments, right? That would lead to buying any other coins in the market at the time. Surely you already know where it is going.
From that people would invest to any other coin in the market after they bought their Bitcoin. It has been like that in the market. Just like the usual market outside, when apples rise in price, there's common scenarios that even oranges and strawberries rise in price especially when they're almost biologically similar to each other.
Smart investors don't just keep and stick to a single coin like having Bitcoin alone. Perhaps, there are a lot of altcoins that are also good for investment and seem profitable which as an investor can't just ignore them but rather get them and buy more. Diversification is a valid reason and investors keep doing this as they wanted to grow their profit. In fact, the market is designed to be like that, buy and sell really happens in crypto. And they'll buy altcoins because of the trust and they believe that the price will rise together with Bitcoin.