Is emigration to the USA out of control?

I just spent a fruitless half-hour on the United Airlines frequent flier website, checking availability of round-trip award flights from BOS to LHR.
Is everybody leaving Europe for the USA? Permanently????
How else can I explain the fact that for every date pair that I tested in August and September, I found one or two eastbound flights with available award seats, but zero (none, nada, zilch) westbound returns?
Of course there is an alternative explanation to “mass emigration”, but I’d prefer not to go to bed in a cynical frame of mind…