Somebody more familiar with Oceanic ATC may be able to confirm this theory.
Most flights eastbound across the Atlantic leave the US in the evening (US time) and arrive in Europe early morning (Europe time). They then start the return journey in the late morning/afternoon.
This "consensus flying" allows most of the traffic to get their reqested route and/or level for the crossing. If it were a free for all with traffic coming and going all day and night, it would be much more difficult to accomodate the number of flights presently flying oceanic without a more restrictive use of levels and tracks.
If you're going against the flow you're more likely, IMHO, not to get the track or level you really want.