How does Gemini choose its altcoins? Are the altcoins they list worthy of consideration over non-listed coins? The list seems random to me...
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Such exchanges want to protect their users or customers.
One of the reasons why Gemini or other exchanges only added few cryptocurrencies or trading pairs to there is the market cap, if you notice the list of tradable coins on Gemini, most of them are a high market cap, trading volume too. So it means that the market is active for those coins, a lot of people that are buying & selling.
And also, some exchanges and maybe Gemini too are very strict when it comes to fundamentals, that's why they don't list random altcoins.