Since the sannin are comparable in strength, and Itachi No-diffed Orochimaru, its just common sense that the whole Akatsuki couldnt defeat Jiraiya
On a serious note, I really dont understand why Itachi couldnt just Tsukuyomi him right then and there, unless he really didnt want to, maybe because Jiraiya was a good asset for Konoha? or because Itachi is still a puppet for the elders