Probably because there are a lot of Schengen Star Alliance airlines within Schengen:

- Lufthansa
- Swiss
- Austrian
- TAP
- Agean
- LOT
- SAS
- Croatia

So Schengen star alliance is really strong, and as these are short haul probably higher frequency than Star Alliance long haul. So makes sense to optimise and invest in the experience in Schengen. OneWorld is different, as there is only Iberia and Finnair in Schengen. BA is probably the biggest ( in terms of frequency ) OW airline, and obviously non-Schengen.

In regards to other non-OW airlines, even if the lounge is branded OW. Doesn't mean that other airlines wouldn't negotiate access. It's not uncommon for airlines to use lounges of competitors, especially when no other options exist. I flew from SFO not long ago, and notice that Qatar uses the Air France Lounge for example.