The thing is... the UK Top Gear was more about cars early in it's life before the presenters had the incredible chemistry they do now. And it worked back then. It evolved to the show it is now due to the presenters and said chemistry, and it works even better now.
the early years of Top Gear was definitely more about cars it had some right dicks as presenters(we are talking 1970s here)and it was proper boring,LOL
In late 90s Jeremy Clarkson who was a presenter left(see below,LOL)
and James May(Captain Slow) took over with other presenters,one i think had to leave awhile later because he got done for drink driving and then mainly because of Channel 5's Fifth Gear
show,they relaunched Top Gear on the BBC in 2002
The explanation is that it was cancelled and renamed as Old Top Gear and then they relaunched the show as ..Top Gear
with Jeremy Clarkson again with newcomers Richard Hammond and Jason Dawe who took over from James May,a series later James May took back his role from Jason Dawe and i think it was from then that as you say they started to gel and it started to get more recognition,especially worldwide...not sure how long Top Gear has been shown in the U.S.
it's funny but i think it was when Richard Hammond had his high speed near death crash in 2006 which gave the show another boost