Because he probably wasn't tribune. The course of Metellus Scipio's early career is very poorly worked out, and several scholars have amended their positions, most notably Broughton. Broughton, in the MRR (Magistrates of the Roman Republic), initially identified Metellus as tribune in 59, but later renounced this view on the grounds that the sources (Cicero and Valerius Maximus) simply don't say that he was tribune. In the Festschrift volume for Broughton in 1996, both Konrad and Linderski further eroded the case for a tribunate and plebeian status. The other major piece of evidence for Scipio's plebeian status was that he was supposedly one of the plebeian pontiffs, but Konrad showed that the long standing assumption that eight of the pontiffs had to be plebeians was based on the college as it existed in a single year, and that it is therefore not good evidence for Scipio's supposed plebeian status. Both Konrad and Linderski also pointed out that the evidence is in favor of Scipio's testamentary adoption (rather than adrogation, the procedure by which Clodius was transferred to another family), and that the fact that Scipio was later interrex is overwhelming evidence against his ever having been a plebeian. The jury's still out on how exactly his career proceeded, but with exactly zero evidence for his tribunate and the evidence overwhelmingly in favor of his having always been of patrician rank it becomes unlikely that he was ever tribune.