A few explanations have been offered on the show, mostly in season 2.

During discussion of the repairs to the Enterprise in "An Obol for Charon" it is noted that one of the most problematic areas is the holographic comms system. Pike tells his engineer to rip the entire thing out and go back to 2D display screens. That may also explain why the bridge viewscreen on the Enterprise is 2D and smaller - it could be a retrofit, a temporary repair even that became permanent.

Enterprise may actually be older than Discovery anyway. It is mentioned in Project Daedalus" that it sat out the Klingon war, far from Federation space, to provide continuity in case the Federation was destroyed. As Discovery was relatively new at the start of the war, an experimental ship with a spore drive, it seems likely that Enterprise is older.

The Enterprise's mission may also explain some of the differences. As a deep space exploration vessel it may have been designed to be more "robust" and easier to repair in the field. Ships of that era don't seem to have replicators, except perhaps for food, so a high tech ship full of complex technology would probably be more reliant on starbases for maintenance.

Which brings us back to the on-going repairs being made to Enterprise during season 2, which seem to involve ripping out problematic new technology that was fitted to it. Being cutting edge has some disadvantages.

A few explanations have been offered on the show, mostly in season 2.

During discussion of the repairs to the Enterprise in "An Obol for Charon" it is noted that one of the most problematic areas is the holographic comms system. Pike tells his engineer to rip the entire thing out and go back to 2D display screens. That may also explain why the bridge viewscreen on the Enterprise is 2D and smaller; it could be a retrofit, a temporary repair even that became permanent.

Enterprise may actually be older than Discovery anyway. It is mentioned in Project Daedalus" that it sat out the Klingon war, far from Federation space, to provide continuity in case the Federation was destroyed. As Discovery was relatively new at the start of the war, an experimental ship with a spore drive, it seems likely that Enterprise is older.

The Enterprise's mission may also explain some of the differences. As a deep space exploration vessel it may have been designed to be more "robust" and easier to repair in the field. Ships of that era don't seem to have replicators, except perhaps for food, so a high tech ship full of complex technology would probably be more reliant on starbases for maintenance.

Which brings us back to the on-going repairs being made to Enterprise during season 2, which seem to involve ripping out problematic new technology that was fitted to it. Being cutting edge has some disadvantages.