I think they try to select real projects. There are so many scams out there, and coinbase is heavily regulated in US.
Also, they are worried about 51% attacks. For example, ETC suffered 3 attacks in the last 7 days. BTG also suffered some attacks in the past and so on.
Personally, I think they could even restrict more the coins that are list. Coinbase is not an exchange like binance, which tries to list all possible coins in the world for money...
They are heavily regulated which means they would need to abide those laws that had been imposed for them to follow or else then their business wont exist at all in the first place.
In regards on listing out coins then they'll surely look for the best ones and delisted later on if they do find out that its a total shit.
I have no idea about their basis and factors for them to list a coin but there's a strong attachment for real projects. Just like Binance, they have a basis for projects that shall bring to their platform.
I havent read it up but you can take up some read here:
https://listing.coinbase.com/When it comes to criterias then pretty sure that it wont really be that simple.