Because that of long time he had amazed them with his sorceries (dia to hikanōi chronōi tais magiais exestakenai autous). Causal use of dia with the accusative articular infinitive (perfect active Koinéš form and transitive, exestakenai). Same verb as in Act 8:9participle existanōn and in Act 8:13imperfect passive existato (cf. also Act 2:7 already). Chronōi is associative instrumental and magiais instrumental case.