Because of his strength. There is no "because of" in the original, and the reading, "his strength" ( עזּוֹ), is doubtful. Several manuscripts have "my strength" ( עזּי), and this reading was followed in all the ancient versions.
Most modern critics prefer it, and translate, O my strength, as in Psalms 59:17. Will I wait upon thee; rather, I will wait upon thee. For my God is my Defence; or, my High Tower (Revised Version).