The layers of backstory, and even creating Saulot and the Salubri Clan(s) all came later on, real world time. So, the real reason, out of game, is because the Tremere had been established first, and the details about what Antideluvian they replaced where not really in place yet. Thats just the tip of the iceberg, and before they swapped Fortitude for Presence. So, the Clan that just publically Diablerized Saulot to gain legitimacy is condemning the Salubri as Diablerists?!?!?!

Keep in mi d, basically all of Cainite Society actually believes/knows the Tremere are wizards and witches in league with Satan/Hell/darkness, not to mention instigating all sorts of trouble and pissing off three entire Clans. (Being that the Salibri at the time have a pretty good reputation almost universally, I don't see anyone actually believing this, but whatever).ДЄfter the Diablerie of Saulot, the rumors about the Salubri stealing souls and being infernal really began, as the Tremere allied with the Ventrue, (one of the Salubri's major allies, wait, so the Ventrue are morons now). Indications are they took some time before deciding on Saulot, then began laying the groundwork for their lies. The original idea was that the Tremere, after diablerizing themselves down to 4th Gen realized they needed an Anti of their own to start being taken seriously. But give the Salubri a Discipline to make the world love them.

The entire Tremere history has always been pretty unbelievable. This is one of the reasons I really hated the changes to the Salubri in V20 DA.

Vampire the masquerade salubri