This fallacious frame rate analogy seems to be repeated again and again.
First of all, it's misleading to state that there is a lack of support for ultra widescreens. If there were no support, the game would either not allow you to play at a 21:9 resolution, or it would allow it while exhibiting various artifacts.
The frame rate is a single axis that can go either up or down. 120Hz is a direct improvement over 60Hz, and the benefits of going even higher, while negligible, are theoretically indefinite. Screen aspect ratio however is not merely something that lies somewhere on a line from high to low. A monitor's width is a relative term, so while it's correct to assert that a monitor is wider than another, it is equally correct to assert that a monitor is taller than another. I'd actually wager that the latter is more true, as many video signalling standards are made to cap at 4K, serving as a reference point to anything lower, but I am getting ahead of myself. Wider is merely "different" rather than "better", and favoring one means that others will be disadvantaged. There are really four routes to take for this:
1. Favor width, and put tall monitors at a disadvantage.
2. Favor height, and put wide monitors at a disadvantage. This is common in games made when CRTs were still dominant.
3. Favor something in between so that no extreme will get a significant advantage or disadvantage. This is what Overwatch is doing.
4. Favor nothing by lifting all restrictions and allowing the player to adjust the field of view as they are pleased.
The last option appeases everyone, but it legitimizes ludicrously distorted projections caused by pathological FoV settings, and many consider it to ruin the competitive spirit.
As for the other options, it really comes down to what works best for most. Because the market is dominated by 16:9, it is what Blizzard chose.
A decent compromise to solve the motion sickness problem right now would be to increase the FoV limit to 120, which would give 21:9 users everything that can be seen in 16:9, while allowing those using 16:9 to see even more than they do now.