The cost.
I could be wrong but I imagine getting natural gas from the US across the ocean and then across France would cost way more than in a pipeline from Russia to Germany.
Again, I could be wrong I didn't knew about Poland. Makes me rethink my stance on the issue.
Now, Poland does it, ok but they signed the deal when their previous deal with Russia was coming to its end. I guess Germany too has a deal with Russia and Russia is not the kind of partner you want to irritate by not respecting the deal you have with them...