What is the reason? Because he should have trusted in Hashem, and did not trust. That is: perhaps you will say that he went to his livelihood, which is also called a mitzvah. That is not so: at the time of tefillah it is not a mitzvah, for he should have gone to pray that HaKadosh Baruch Hu give him his livelihood; and he did not go to pray, so he did not trust in Hashem.