Why would my boyfriend not delete all messages with his ex?

We broke up for 4 months and tried to move o man she had a girlfriend for a month or so and he didn’t delete all the messages. I shouldn’t have went through his phone, but it was just there. I’m not going to tell him or bring this up. I have already done it before and told him and he wasn’t too happy, and can’t blame him because it looks like I don’t trust him. Also, he seemed to be more flirtatious with her than me so now I want to die, lol. But it’s my own fault.