One reason the conspirators might not have trusted Cicero with an assassination was that he and Caesar were political enemies, but were often on good terms socially.
Cicero's nephew the younger Quintus was a Caesarean.
Caesar and Cicero had a wide circle of correpsondence and, although they were wary of each other, they actually seemed to have gotten along on a personal level. Caesar felt it worthwhile to try to get Cicero to reamin neutral in the Pompeian conflict, sending at least two letters to that effect. When Cicero wrote a panegyric for his deceased colleague from theSenate Cata, Caesar reposnded with an "AntiCato" but took no personal action against Cicero. Quite the contrary. It remained a literary and philosophical dispute.
Caesar is also known to have taken time while on campaign to write Cicero about the death of Cicero's daughter Tullia.
And less than three months before the Ides of March 44 B.C., Caesar was Cicero's dinner guest at Pulteoli.
"There was no serious talk, but plenty of literary. In a word he was pleased and enjoyed himself," or so Cicero reported to Atticus (Att. 13.52.2).
It's hard to know WHAT to make of their relationship. And it may have been just as difficult for the conspriators as it is for us today. Another factor may be the age of teh conspriators versus Cicero. Cicero was born in 106 BC and Caesar in 100 BC. They were of the same generation more or less. And Cicero may have looked down a bit on the younger generation, as well. after teh fact, he decribed teh assassination like this: "The deed was done with the courage of men, but with the planning of children" (Ad Att XIV, 21). Brutus was a full generation younger than Cicero. In the period before the Ides of March, the older generation, represented by Caesar and Cicero, might have seemed too chummy perhaps.
After Caesar's death, of course, Cicero jockeyed to protect and justify the tyrranicides and wrote in their favor. But before that he had usually counseled restraint.He might have seemed to unlikely a supporter to them before the fact.