It is very risky to bet all in Europa League. Even if they bet it and focus all resources for Europa League, they might fail too. We already see in this season how easily Barcelona can be beaten by small and medium clubs. Since they are falling down in quality, opponents don't need to be Real Madrid, Bayern or top notch clubs to beat Barcelona.
I agree with you, besides, Barcelona enters this tournament from the playoff stage, and here traditionally any small team always has a chance to knock out a stronger one simply due to the fact that this is not a round robin tournament, but only two games where the influence of chance is great.