I haven’t seen anything official anywhere so these are just my thoughts.
Twitch assumes that Extensions are being developed by “businesses,” therefore they treat us as such. You usually would not expect a business to be running a test stream during the weekend or off hours, therefore they avoid reviewing during those times. This really only pertains to overlay Extensions, but I feel Twitch would want a near identical process for both overlay and panel Extensions. Independent developers might also start to complain that Twitch reviews panels on weekends but not overlays.
From my understanding, Twitch acquired Curse Inc. in August 2016. Under Twitch, Curse has been tasked with revamping the Twitch developer website. In addition, Curse, through their Curseforge service, had a system to review and approve uploaded game mods, so it made sense for Twitch to task them with reviewing Twitch Extensions since they are basically mods for Twitch channels. I have no idea if Curseforge reviews mods on weekends, but if that is the case, perhaps weekend reviews are possible in the future, depending on the community’s response to Extensions.