Indeed, CO are still doing BRS-EWR daily direct- you have to remember BRS has about 3x the passenger throughput of LBA though, and the next closest 'big' airport (Heathrow) is farther from Bristol than Manchester is from LBA.
Having said that (and having lived in both Leeds and Bristol and used both city's airports), Leeds and Bristol are similar in terms of the kinds of companies establishing themselves in the city, and the BRS flight does very well from incoming American business travellers, so I would imagine a Leeds service would too.
It all depends on the carrier and frequency though. If it's not an airline that affords easy connections all over the USA (*A or Skyteam member), and the route is not frequent enough, it will fail.