Rajasic austerity has a hidden audience. It is performed so that people will offer respect, honor, and admiration. The person fasts or undertakes some discipline not for inner growth, but for outer recognition.
Imagine someone who undertakes a long public fast so the neighborhood will call them a great devotee. The practice looks identical to sattvic austerity from the outside, but the engine running it is entirely different. What fuels it is the craving for applause.
Krishna calls the results of such austerity "chalam" (unstable) and "adhruvam" (fleeting). As long as the praise flows, the practitioner feels good. The moment the spotlight moves on, disappointment sets in. Austerity that depends on an audience cannot last.