We have no idea. So far as we know, when a clump of gas collapses, some of the matter ends up in the central core, the rest in an orbiting disk. This disk could, in turn fragment into a second core which could become a second star if enough matter is involved; it could also fragment into numerous smaller bodies, comet, meteors, planetoids etc which form planetary bodies if the conditions are right. We do not know the limits to this process. If you only have enough mass in the collapsing core to form a brown dwarf with a mass of 40 - 80 times Jupiter, will the remaining material be present in a circumstellar disk which could further fragment? Or is there a threshold for the remaining mass in the disk below which the planet formation process is inhibited? Planet formation is presumably not a very efficient process so a lot of stuff might be needed in the disk. Also, the known brown dwarf candidates are all in binary star systems, mainly because they are easier to find there. If there are no isolated brown dwarfs, then it may just be that it is hard or impossible to form such low-mass stars outside of a binary system. We just don't know enough about brown dwarfs yet.