A widespread conception is that Brahmins were primary regulators of caste relations in traditional village societies. It's true their ritual status was highest across geographies but the most decisive elements in regulating caste relations and behaviors were not always Brahmins.

The primary guardians usually were what M N Srinivas called the dominant castes which were broadly defined as those castes which had ample land and sizeable population along with a high ranking in the caste hierarchy.

While Brahmins were the dominant caste in many cases, in many others they were not.
This was especially true in the south.
As examples of dominant castes one could think of Reddys and Kammas in Andhra, Gounders in TN
AJGAR castes : Ahir, Jat, Gujjar, Rajput in North.

These were the castes which decided what kind of clothes and headgears a certain caste could wear, what kind of material possession they could have etc.

And these were the castes that administered punishments when a certain jaati went out of line.

So the question is why are Brahmins seen as the primary villians in debates about the 'oppressive nature' of caste system ?

Sure there is concerted propaganda and all but another reason could be that even when Brahmins were not the dominant caste, their support was sought by the dominant castes due to their high ritual status.

It was always good for dominant castes to have Brahmins on their side for it the Brahmin support provided greater legitimacy to position and actions of the dominant caste. And for Brahmins too having dominant caste on their side made sense.

While Dominant caste changed from one geography to another, the Brahmin was present everywhere most as an ally of the 'oppresive' dominant caste.
Even though Brahmins as a group consisted of many sub groups that had absolutely nothing to do with each other.

However, due to their pan India presence they became a national symbol for excesses of jaati vyavastha even though in places like Punjab their social status was quite low.

And they were often ridiculed by dominant castes like Jats who saw Brahmins as their servants. This was also true for many societies in South.

One wonders how different the caste narrative would have been today had there been one or more materially well off landowning Pan India shudra caste.

Would liberals marxists etc still have attacked the caste system as bitterly as they do now ?