Bear in mind that while there were only 5~8 jobs accepted endgame, WHICH of those 20+ jobs were among the 5~8 has varied wildly over time as SE added encounters that favored certain jobs, made tweaks to existing jobs, and so on.

There was a time when WARs were the most highly favored DD. Now you hardly see them at all. NIN was once the most favored tank. Nowadays, people won't even use NIN as a subjob. Even mainstays like WHM had their time out of the limelight, when RDMs were favored for their longevity. While it's true that there's a handful of jobs that never really had a chance as serious endgame raiding jobs (BLU, DNC, BST, PUP), they kinda made up for it by being superb solo jobs.

As Ehayte says, even with the small selection of jobs we have now, favoritism is still happening. It's pretty much inevitable. Better to have a wide range of classes, then - variety is the spice of life!