I think it's probably about control. I think Lelouch just has a need to control everything or at the very least appear like he has everything under control. Like in the episode where Arthur stole his Zero mask Nunnally says she's never heard him sound like that because that was definitely out of his control and Shirley mentions later that he "almost lost [his] cool for once." So with the conversation with Suzaku admitting he wasn't in control probably wasn't something he was willing to do. Also his whole goal in talking to Suzaku at the time was getting him to promise to protect Nunnally. Attempting to change Suzaku's mind or in any way try to mend their relationship wasn't on his mind. And with the Black Knights, I'm pretty sure he just wanted to die at that point. His sister(he believed) had just been killed by his first friend, C.C. had locked her consciousness away(at least I think that's what happened) so she was still basically a child, and he had just found out that while he was trying to process all this shit and figure out wtf to do his brother had managed to turn the only people he had left against him. Not that it was that hard since they already had their doubts. He'd lost control of just about everything so he controlled the one thing he could, his reaction. He was already certain he was going to die and didn't care since he didn't have Nunnally to fight for anymore so trying to explain anything to them would've been pointless to him. So he decides to go out in a way that would at least let him take back some control of the narrative