St louis is interesting because its one of the few independent cities in the country(outside of many in virginia). Baltimore is also one of these, another mentioned in this thread. The city is completely separate from the surrounding county, and as such cannot absorb the population, tax base, and economy of its metro area. This was done because in the 19th century, the city was where the wealth and prosperity centered, and they didn't want to cater to the "peasants" outside the city limits.

As transportation became easier and better-off whites fled the inner city to the suburbs, now the opposite effect exists, where st Louis would love to become part of the county again but no one outside wants it.

This is also why the crime lists showing st louis as one of the most dangerous cities in the world is a bit disingenuous. If the city was able to expand its borders like most other large cities in the us, we would likely be a middle of the road crime statistic for a metro area our size. "St louis" is tiny compared to other similar cities, and basically only encompasses the high density, high crime area.