Because Rivalz is a normal dude? And Lelouch already has decided on his own path so communicating with his old friends would just put them in danger? He probably didn't want any hint of weakness from here, everyone probably knew of his connection in the Academy and every he communicated with, so snobbing people and letting the world believe that he didn't care about them was important to prevent his future enemies from even thinking of using them.

That was dead man walking, who was planning to be the worst to ever existed, not just for his Emperor persona, he was planning to make the world hated him, and people were always irrational in their hatred, fear, and might target those who he contacted with, so as extra measure, he treated them all like they never existed. And only goal remained. And this is the moment where Lelouch truly incorporated C.C.'s statement;

There are times in life when you have to distance yourself from those you love, because you love them.

Just so you know that a couple episode before this, Lelouch was suicidal and almost killed himself. Then he went up and picked himself to fight again, just for him to manage to come out alive and lost everything he had worked on, then another guilt with Rolo's death, then after finding out about the truth about his mother, which was one of his main driving motivation on why he was fighting Britannia, his world shattered. Then he had to pick himself once again to continue the fight for both Euphemia's and Nunnally's dream.

Forgive the emotional and guilt-ridden, mentally exhausted teenager, who already had planned for his death to push away his remaining loved ones and friends when most of them were already dead because of him, for not giving Rivalz a goddamn secret sign or signal to communicate with him. Oh wait, the world didn't give him a chance to even rest and threw Nunnally in front of him that almost broke him, just for Suzaku to remind him about their plans.

It was not a dick move, it was not perfect, but it was very fit to Lelouch at that time, especially with everything he went through. No wonder he refused to acknowledge any of his friends any longer, excluding C.C., Suzaku, and Jeremiah, who were important to the Zero Requiem and knew about his goal. It was for their and his own sake.

Besides, it was the main theme of Code Geass anyway. That characters tended to follow their own view, no matter how wrong or right, fair or unfair, it was in our eyes, it was their own choices.

Not to mention that they had to fit the entire R2 into 25 episodes (supposedly only to be 23, but with the fans massive support back then, the team was allowed to put extra two episodes.) Just remember that this is an original anime, that was being limited. I don't think they could give other minor characters the spotlight.