In recent decades they mostly got sidelined by "traditional" side entry bags, be they mummy, rectangle or other. There have been several more fringe bags that held on to the tradition, Golite's adrenaline bag, Wiggys military, a few higher end down options and some mobility bags such as the Exped dreamwalker and Feathered Friends Rock Wren.
As of a few years ago center zips started gathering steam, and have now officially gone mainstream:
Both The North Face and Mountain Hardwear now have center zip bags, the Inferno and Hyper Lamina respectively.
I probably first noticed the trend when Sierra Designs released their Mobile Mummy and Kifaru switched their contentious Slick Bag to a center zip, but looking back there have been more trickling into the market recently.
Whether the change can be attributed to better technology (improved baffle designs?), a more cut-throat market for ruthless efficiency, or simply a desperate quest for newness is beyond me but I am very interested to try one. I have in the past used a zipperless bag I quite liked and my return to a more traditional mummy has left me underwhelmed so a center zip is next on my list. For one, with an adequately long zipper (not the short 1/4 zips on the ultralight bags) entry and exit may be easier, I think their are gains to be had in hood design, and half-asleep middle of the night adjustments may be more intuitive.