It is simple. The public enthusiasm for ICOs have died. The number one priority of exchanges is to profit. And they profit through trading fees so they need trading volume, lots of it to be profitable.
Since no one is interested to buy ICOs after exchange listing, why bother to list? It will just put a strain on the exchange platform. So, they are now more choosy on which to list.
And yet the influence of regulatory authorities is having an effect. All the same, exchanges are primarily interested in profit, and secondly, they are interested in the legality of transactions (at least something like this). Now, unfortunately, exchanges can be held liable if they are involved in fraudulent fundraising.