Answer: Because the Senate wanted Pompey as only consul, excluding Caesar, who was dreaded due to his great reputation.
Step-by-step explanation:
At the end of the First Triumvirate, the Senate preferred Pompey, while Caesar, who had an outstanding support of the people, was rather feared. Apart from the fame among the people, he had the faithfulness of his glorious legions.
Caesar’s great reputation led to the demand of the Senate that he give up the command over his army, what Caesar defied and entered Rome along with his legions. This was considered a coup d’état and the Roman Civil War began between Caesar and Pompey, who had the support of the Senate.