As an American that has never driven an American manufacturer's 4WD (i.e. Losi) I would like to respond to the statement, "Not enough of your drivers were game to try other products." I've always raced 'foreign' 4WDs.
It has been my experience in the past that raceable 4WD kits used to run up to 3 times the price of a good 2WD. We are talking in the neighborhood of 450-600 US Dollars! I do not know whether this was due to import costs (from Japan, UK, <insert country here>) or simply due to lack of interest (and therefore low volume).
For the beginner/hobbyist cost is often a large factor as to what class(es) they choose to race. 'Club Level' racing is where world champions start. Also, I think 4WD has a stigma of being "hard to wrench on" due to the more complex drivetrains. I think Losi has done a good job trying to stamp out this 'rumor' and I believe that is part of their success with 4WD.
Also, until very recently (maybe 6 yrs) 'foreign' parts were hard to get in the USA. Tamiya was imported by MRC (sucked) and Yokomo...wasn't really available. Now that TamiyaUSA and YokomoUSA exist, parts for these kits are much easier to get.
Tamiya doesn't have a serious 4WD offroad kit nowadays. They just cannot compete in 'big' races. Hopefully that will change.
Yokomo does have a serious 4WD kit, but I think there is a serious lack of hobby shop support here in the US.
I believe some of the resistance to other kits relates back to nationalism. I'm sure the same could be said in the UK (for Schumacher) or Japan (for Yokomo). If it is built 'locally' parts and support are bound to be easier to find.
What is popular at the club level is what wins and what is affordable and what is easy to repair.
Currently, Losi has all three of these things covered.
As a side note: I think if the BJ4, X5, Durango or the Academy 'SB Sport' could capture a National or World title, their sales would skyrocket.
PS--This is not ment to be a flame in any way ... it is just my .02 USD