The reason is that Sánchez is attempting to form a government with the support of Catalan separatist groups.
The issue is that the separatist groups, Together for Catalonia (JxCat) and the Catalan Republican Left (ERC) have demanded an amnesty for several hundred Catalan politicians and activists who face legal action mainly for activities linked to a failed bid to secede from Spain in 2017.
The Sánchez administration have already issued pardons for nine of the leaders who were jailed for their part in the failed secession bid; they subsequently revised the penal code to eliminate the crime of sedition and alter the crime of misuse of public funds, changes which benefitted Catalan separatist leaders.
Among those who would benefit from a change to the law is Carles Puigdemont of JxCat, the former Catalan premier who led the independence bid six years ago and who has been living in Belgium ever since, out of reach of the Spanish judiciary.
Naturally, the PP are opposed to all of this as are a great number of Spaniards. They would much prefer another election in December to try and form a Government without the need to resort to such shenanigans.
They definitely don't want to open the door to another attempt at a referendum which could see Catalonia separated from Spain.