User blog comment:Supahbadmarine/Official Canon Policy/@comment-10514705-20130223075041/@comment-10514705-20130223194514

"The Salamanders were always the smallest of the First Founding Legions, owing to their indoctrination procedures and the large number of Salamanders Space Marines slain at the Istvaan V Drop Site Massacre during the opening days of the Horus Heresy. After the massacre on Istvaan V, the Legion's numbers were severely reduced and, as such, no Successor Chapters were formed from the original XVIII Legion during the Second Founding. Although, Imperial scholars have pointed out the similarities in the physiques, markings, and tactical dogma of such Chapters as the Black Dragons and the Storm Giants which may be Salamanders Successor Chapters created at a later date from the stockpiles of their gene-seed."

That essentially is the reason for it. The Ultramarines spawned so many successor chapters because they were the largest of the Legions. The Salamanders have not been so fortunate in having stockpiles of geneseed.