Maybe previously they could survive off BTC alone, and it benefited them to focus on the healthiest currency, but now BTC isn't doing too hot, so they feel the need to cast a wider net so they can get a head-start on the next craze, should it come?
Volatility is an exchange’s friend. Out of the entire cryptocurrency space, Coinbase is the one that makes sense—selling shovels.
To the extent that the investment philosophy leans more towards "HODL" as prices decrease, trading volumes will go down, and trading volumes predicate volatility.
This happens in all markets. Precipitous price drops scare away buyers and trigger loss aversion (and an aversion to marking down positions) in sellers. That leads to the market transitioning to an illiquid regime and non-viability for exchanges.
I don’t think we’re there yet with cryptocurrencies. (Beanie Babies took over a decade to resolve.) But it may be what Coinbase is guarding against with this move.