I thought the whole point of them being in the Big West was to cut down on travel costs for Olympic sports
Well, yes, but only kinda. We tend to overrate costs as if airplanes are like taxis. The cost of flights, hotels, busses and food are basically ridiculously similar regardless of where you're going once you hit the airport. But the more time you spend traveling, the worse your legs and mood are; so if you're Hawaii and have to fly through LAX, San Diego and Sacramento to go anywhere else mainland, why not just play the 10 schools near LA, SD, SAC in conference?
It's not so much that Hawai'i was like "Let's leave the WAC for travel savings" as it was "The WAC radically changed over two waves of downgrades and instead of being a good football league with BYU, Utah, San Diego State and UNLV, we're in a league that no longer has FBS football and has Seattle U, UT Arlington, Texas State and Louisiana Tech.
There was no upside to being in the WAC over being in the Big West. There was a large upside to the Big West over the WAC from 2012-present: The logistics of travel were just better in the Big West. Plus they had more history in non-revenue sports with BWC schools (and volleyball IS a revenue sport in Hawaii).
There's once again an advantage to being with Air Force, UTEP, Wyoming, New Mexico, UNLV and Nevada (some being schools they went to the WAC to be with, or voted to add to the WAC).
It's mostly about football security. They just can't afford to be an independent. It's less about what they gain joining the MWC but what they avoid potentially losing by not joining the MWC.