The latest episode of Star Trek: Discovery, "Point of Light," may have just explained why the show seems so much more advanced than the Original Series. When Star Trek: Discovery first began, it didn't take long for Trekkies to note that the ship seemed to have technology far beyond Captain Kirk's Enterprise in The Original Series, which is ostensibly set a decade later.
Some of this can be explained away by arguing that Discovery is a science vessel, and more likely to have experimental tech - like the Spore Drive itself - that may prove problematic and be discontinued. But in truth, it reflects that Star Trek has always been a futuristic series. Back in 1966, massive viewscreens and hand-held communicators were the stuff of science-fiction. Nowadays, they're commonplace, meaning Star Trek has to embrace new technologies in order for its science to still look advanced.
Producer Aaron Harberts has promised that Star Trek: Discovery season 2 will begin to address some of these continuity issues. "Point of Light" has begun that process, suggesting that there may be a reason the Enterprise never adopted the holographic communicators we see in Discovery. In one key scene, Captain Pike contacts the medical facility treating Spock, and engages in small talk for a moment. He's rather better at it than most Starfleet captains - it's hard to imagine Kirk ever bothering to take the time for chatter like that - and as part of the conversation, viewers are told that Pike is quite an old-fashioned captain. He's one of the few people to still to use the viewscreen. The conversation seems to imply that the holographic communicators are fairly recent tech, which would explain why even the Discovery has both systems and not just holograms. Pike is one of the few people who prefer to stick to the tried and tested screens, so perhaps James T. Kirk took a similar view when he became captain of the Enterprise.
Fashion is important, even in matters of science. Just because a new technology becomes available, it doesn't mean it will be widely adopted (look at Google Glass, for example), and just because it has been embraced by society, it doesn't mean trend-setting won't lead to it being abandoned again. As Captain of the Federation flagship, Pike's everyday decisions would carry weight, and could easily influence other Federation captains to ditch the holograms over time as well. Kirk, of course, becomes even more famous than Pike; if he had the same preference towards the viewscreens, it would be even more influential. If most captains were choosing to use the screens, little by little Starfleet Engineers would begin to wonder where it was even worth installing the holograms in the first place. By the time of Star Trek: The Next Generation, they could have been dropped completely.
Of course, it's also possible that other factors led to the holograms being abandoned. Perhaps they proved to use too much power at critical times, perhaps they malfunctioned, or perhaps an alien race worked out how to hack into them and use the holograms against the Federation. In Star Trek: The Next Generation, the Holodeck proved particularly vulnerable to sabotage, and it's clearly a related technology. Would it really be wise installing such a system in key locations such as the Bridge or the Captain's Ready Room?