Mercer Island exists and has political clout (even though they tried to use that clout to force Sound Transit to not use the express lanes on I-90) and would demand a station no matter what.

Running across a floating bridge is hard enough and the Governor Albert Rosellini Bridge (SR 520 Floating Bridge to the Yanks) is the longest in the world. Going across the dual-named bridge that carries I-90 would be shorter and was seen as easier back then.

You don't have to turn around to serve Bellevue and then Redmond. Coming up from I-90 you hit the big part of Bellevue then you turn right and keep going northeast to Redmond. Coming from SR 520 you'd go south into Bellevue then have to U-turn to go back to meet BelRed and Overlake.

At the time, we didn't know when, if ever, the SR 520 bridge would be replaced. The previous span was a nightmare. It had a movable bit in the middle, the road deck sat at roughly water level, and would routinely sway so hard in strong winds that the bridge would have to be blocked off so the midspan section could be opened to relieve stress and prevent the thing from sinking. The I-90 bridges have no such issue (considering the damn thing had already sunk once in the 90s). There was no way to use the previous SR 520 bridge for light rail, though the new one is built to support it.

You also catch a lot more potential riders in station areas around I-90 and South Bellevue. The Points cities are basically HOAs with delusions of grandeur. There's no hope they'll gain more people in any of our lifetimes, meanwhile the areas the 2 Line passes through are already fairly populated by Eastside standards and have room to grow (also by Eastside standards).

Meanwhile, the 542 and 545 are comparatively very fast.