Prestigious startup accelerators in the U.S.--like 500 Startups, TechStars and Y Combinator--generally "select against" solo applicants. But why reject entrepreneurs trying to follow in the footsteps of lone founders like Sara Blakely (Spanx) or Jack Ma (Alibaba)?

"We believe being a single founder is one factor that makes it more difficult to succeed… [because] there is just so much to do at a startup," said Y Combinator partner Jessica Livingston, who estimates that fewer than 10% of companies accepted...