Because that was the last wish of a dying man.
5 seconds sound clip from the Gladiator (2000) movie soundboard.
You can hear this line at 02:10:09 in the Blu-ray version of the movie.
Quote context
[...]
- The time for half measures and talk is over, Senator.
- And after your glorious coup, what then?
- You'll take your 5000 warriors and leave?
- I will leave. The soldiers will stay for your protection, under the command of the Senate.
- So, once all of Rome is yours, you'll just give it back to the people? Tell me why.
- Because that was the last wish of a dying man.
- The fate of Rome, I leave to you.
- Marcus Aurelius trusted you. His daughter trusts you. I will trust you.
- But we have little time. Give me two days, and I will buy your freedom.
- And you, you stay alive, or I'll be dead.
[...]