There are fewer westbound routes although I suspect that a lot of traffic transfers at DUB & AMS - & that IAG plans to increase the former. This may be because the ME3 are more concerned about volume (you could argue ad nauseam about subsidies) the US & UK carriers need high yield traffic and it is probably the lack of high yield traffic holding back MAN - LAX/SFO. As said probably less of an issue for VS than BA and not one at all for DY or TCX. There are 3.1m pax p.a. to the US from regional airports but many go to the leisure destinations of LAS & MCO.
I think your post was talking about regional traffic in general, as opposed to MAN specifically, but a mildly interesting fact for this Summer's schedule from Manchester:
There are 51 weekly services to the Middle East ex MAN during August, and 101 (almost double) westbound to the US and Canada alone.
SFO seems MAN's biggest gap in North American coverage in terms of direct destinations - but with LA only just re-introduced at 2 x weekly, and numerous indirect options, I wonder how likely that is in the near future?
(NB Excludes Mexico/Caribbean - and PK if they still operate to NYC because I believe it's still just one way UK-US.)