Coinbase is terrible! Someone help me understand...

How come every time there is a buying opportunity for Ethereum, Coinbase shuts down that option...they well let you sell but well not let you buy in a sell off...I'm confused to why they operate in this manner??? I understand with all the transactions that the network can get backed up, but then wouldn't that affect the sell option also? Send they want your ETH at a low price, but you can't acquire any at a low price...what an I missing?