Those who’ve served any amount of time as an elected magistrate [must] become senators.
… ex-court magistrates cannot join the plebiscites; they are either senators or part of the higher-class assembly [which is not detailed on this page].
Jason never made his plans explicit, but everyone knew that he would disgrace his father and join the plebis after discharging from the Twelfth Legion. Even after the Adventus, the only way to prevent his shame was through magistracy.
Reyna’s first term had been a consensus, wherein the legion voted for no other praetor, thanks to Lupa and favourable divinations—and, of course, her own dignitas. Every term that followed was a consensus primarily because she and Jason grew close; the legion wished to force Jason’s hand, hoping that his care for Reyna would make him step up to save her from solo praetorship. There’s a maximum of two praetors allowed (which proved a loose law, considering The Son of Neptune), so Reyna’s solo praetorship also ensured room for Jason’s praetorship—should the legion finally find an opportunity to skip the election process and lift Jason upon their shields.
Because he’d been appointed praetor, Jason was barred from effectively using his auctoritas to empower the People.
n.b. In canon, Jason has served more than enough time (twelve out of the minimum ten years, at the start of HoO). In NRHD, he was excluded from the minimum probatio age (12) by an augury: he skipped the probatio period and became a legionnaire when he was 7—three years after being delivered to New Rome on Lupa’s back. By the time he was forcibly elected praetor, he’d been serving for eight years; two more without holding office, and he could’ve left (accurately expecting an honourable discharge) and joined the plebiscites.