The Convair 880s and 990s had several disadvantages, (over the rival Douglas DC-8 and 707) the most important was passenger capacity. This was the largest contribution to its failure. The 990 only had seats for only 106 passengers. While the Boeing 707 could carry 202, and DC-8 (before the super 60 series) 179. So it probably was not as cost efficent to operate.

--Motov