The article spends a lot of time painting it as mystery before this seemingly obvious paragraph: "Portegies Zwart favours a different explanation, where Jumbos form in the same way as stars, directly from the collapse of a cloud of gas. Known as in-situ formation, this would require us to rethink how low the density of a gas cloud can be in order to trigger such a collapse. But for Portegies Zwart, "I think in-situ formation is the only one in which I don't have theoretical problems," he says. "It is the most promising."

Why would a nebula not produce _every_ possible size of object over its billions of years?

Maybe this is how stars actually form, gas accretes into chunks of icy matter similar to what happens in Saturn's rings. Then those small objects gravitationally meet and sometimes coalesce into moons, then small planets, big planets, brown dwarves etc. finally when enough stuff has coalesced, the gravity is strong enough to cause fusion and boom you have a star.