It is the lowest quarter-final representation from France in Heineken Cup history.
Sowerby, the South African second row, said: "It is very difficult to explain why we are the only French team through.
"It is disappointing, because you are talking about some big clubs with big budgets.
"We will go to Cardiff now and hope to pull something off there.
"They have improved a lot since we played them last season, and are one of the form sides."
Toulouse lost to Munster in last year's final.