Because of the channel hopping I presume. They can't keep tabs on who's sending what, where they're sending it to or where anything is coming from. Once you've entered it in a KYC ish manner via the main chain your money can be subsumed.
Anyway this is only an initial thought but quite a few people have voiced it.
Exchanges don't need to keep track of the whole blockchain and each Lightning transaction though. All they do and should care about is coins and fiat entering and leaving their system. I don't think KYC / AML regulations require anything beyond that. Otherwise you wouldn't see the likes of XMR and ZEC on Kraken, for example. But maybe I misunderstood your concern.
It is just like trying to switch all the bitcoin users to BCash.
I am not saying that it is impossible, but you need to think about this! The exchanges and massive manipulators are NEVER going to allow this to happen.
Why shouldn't people switch to LN and what do exchanges have to gain from preventing LN adoption from happening? Everyone profits from low transaction fees. Looking at the big picture even miners stand to profit from LN in that it will help Bitcoin's adoption and long term sustainability.