Yes, only 50,000 of the Memphis metro's 1.35 million live on the AR side (less than 4%). Though another bridge isn't likely to change that very much.
And while a much larger percentage of St. Louis' population is on the IL side, it is also spread out a lot more, Columbia to western Alton is over 35 miles. Further, the St. Louis CSA is almost 2.5 times as big as the Memphis CSA.
Quad Cities are smaller, but the river is not near as wide, they could build cheaper lower or swing bridges instead of high clearance, shorter spans, and the population is also pretty evenly split between both sides.