Unfortunately, protest songs could absolutely be commercialised.
While not exactly protest songs, the grunge movement of the 90s was a reaction to what was seen as shallow, packaged pop music of the late 80s.
Once their snouts smelled profit, the usual middlemen rushed in to wrap that discontent in plastic and sell it to kids at Walmart, chewing up musicians along the way.
I think it's simply that protest songs are more difficult to wring profit from compared to creating pop music using the formula that sold the most last time.