Low-rated corporate loans are having a banner year. Wall Street is trying to find more ways to sell them to ordinary people. 

At least four asset managers, including BlackRock and Nuveen, have recently asked permission from the Securities and Exchange Commission to launch new exchange-traded funds of collateralized loan obligations—securities made by bundling junk-rated loans together. Those will join about a dozen CLO ETFs that have already entered the market in recent years and that now have about $16 billion in assets under management.