He rode high in a weak field Giro one year working for Egan and that made him a sure thing for a podium contender at the TdF in the eyes of the British centric media. It's easy to pinpoint riders due for a breakout ride because they are really strong in a race like Pais Vasco or the Dauphine, but the pace at the TDF is a different beast and you have to be able to recuperate every single day. Also the Ineos factor seems to push the expectations higher for riders like him, as though he's an automatic GC contender at the Tour since they were the dominant team of the 2010s in the TDF.
That's not to say that Martinez won't contend at the TdF in the years to come, but he hasn't really shown anything at this point that should've elevated him to the podium shortlist over a proven rider like G Thomas, or add an extra star next his name in those prediction things that everyone loves to use to rate the contenders ahead of big races.
Again that's not to say he won't be a TDF contender next year, or in the future, just Ineos has yet to crack the Slovenian code. Bernal won the Giro last year and is a great rider, but he was up against the likes of Damiano Caruso, Simon Yates, Vlasov, and Hugh Carthy. At the Vuelta, he finished 5th behind guys who had just come from the Tour. So that's not to say Martinez's fifth place in that Giro wasn't great, but it wasn't against the guys who would be competing for the podium of the TdF. Most TdF breakthroughs have proved themselves in the Vuelta the last 2-3 years.