Those coins are having a chance to be considered as securities that can't comply with US regulation.
But the delisting of those coins didn't give any impact even that already happened. People are feeling difficult when it comes to dealing with US security exchange site like polo due to the uncertain regulation.
The delisting of coins gave a little decrease at the trade volume of polo but it doesn't give any impact to the daily volume of the coins were getting delisted.
I'm not sure if folks are even reading what was delisted before responding here. The coins were delisted due to low volume because they are in really weird pairings, several against Monero. It had nothing to do with being a security, unless you consider LTC a security (it was one of the pairings delisted).
This is the list: LTC/XMR, DASH/XMR, ZEC/XMR, MAID/XMR, NXT/XMR, BCN/XMR, LSK/ETH, GNT/ETH, MANA/ETH, QTUM/ETH, STEEM/ETH, OMG/ETH, LOOM/ETH, SNT/ETH, CVC/ETH, KNC/ETH, GAS/ETH, BNT/ETH, LOOM/USDT, SNT/USDT, KNC/USDT, BNT/USDT and FOAM/USDC.
The question shouldn't be why those pairings were delisted but why did they even exist in the first place. Common sense tells you they won't have much volume.