I think there's a few things really. Firstly, French club and international rugby is in a stronger place than English rugby at the moment (Exeter aside), which is compounded further by the lack of salary cap in France, meaning the likes of clermont can have the back line they had on Saturday.
Realistically the top 6 in France would beat any English side at the minute, bar Exeter, and that showed this weekend, although French sides are always fast starters in Europe, the likes of clermont, lyon and toulouse normally fall apart a bit in the knockout rounds.
There's also the fact the French sides started their season at the start of September, they've had a lot more match practice and so are going to be a bit sharper.
Going to be very interesting to see if they can keep it up as the tournament progresses as the champions Cup needs top quality French sides at their best