I don't play on Global anymore since they utterly failed to handle PR after the banning disaster, so I have no dog in this fight. However, I may be able to provide a partial answer.
Quote:
they seem determined to only open Surge during the 4 hour evening window
The first reason is to artificially build interest in the same way fast food chains roll out certain low-quality sandwiches or desserts only at certain times of year. If those items were on the menu all the time, nobody would care because they are poor quality. However, when the company builds artificial scarcity into their product, they exploit a bug in the human psyche that makes us want rare things. It didn't really work in the case of Global's Zoom, but it's a commonly-used tactic.
The other reason may have a technical and financial basis: A lot of systems run on rented, dynamically-created servers these days. Major websites can automatically spin up more peer servers during peak traffic times and spin them down when traffic subsides to save money. If Global's infrastructure runs on dynamic servers, they can save on operating costs by spinning down the unused ones for several hours per day.
I don't expect official answers from Global about the reasons because they proved that they can't even keep accounts safe or assure people that their balances will be available, so these are simply reasonable guesses.