A couple of things:
Lufthansa and Air France are the flag carriers of the nations with the largest stake in Airbus. I'm not accusing anyone of sleazy tactics or bribery, but I'm sure it helped push them towards the Airbus offering.
Wasn't the A340/330 series offered earlier than the 777? My memory is hazy but I think Boeing waited a few years before countering with the 777 offering, and there was a 2 to 2.5 year difference in service entry dates. Carriers who wanted non-747 longhaulers in the early 90s timeframe chose between the A340/330 and the MD-11. And we know who won THAT one
(no offence to MD-11 fans, I like it too).
The 777 has a greater payload than the A330/40 but it sounds like there are market situations in which the extra lift is not necessary. It seems as if NW took a sincere look at both the 777 and the A330 as a DC-10 replacement on the North Atlantic and concluded that the Airbus offered the best cost for the loads they expected, reducing the chance of 'eating' seats, as I believe on a per-plane basis the A330 has lower operating costs (would make sense anyhow). There was a post here a few months ago which offered a link explaining NW's rationale -- worth a look (sorry, don't have the link on hand).