I'm going to play devil's advocate here and say that maybe it's because LN-related deposits/withdrawals simply doesn't have that much demand as of yet. Whether we like it or not, the crypto normie world barely uses LN, it's mostly the Bitcoin enthusiasts that are using LN as of today — in which the actual Bitcoin enthusiasts are only a very small percentage of total crypto exchange usage.
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I don't really agree here. I think that if services and exchanges were to adopt the lightning network, more people would start using them especially now with many wallets (Exodus, Wallet of Satoshi, etc.) supporting it and making it easier to use. Who would want to pay 10-15$ extra for withdrawing their funds when they can do it for a couple of satoshis instead?