Because very tickets being sold in these sections.
Premium fare (above ME fare)+ Flexifare + no telescopic benefits for short distance make short distance Shatabdi travel very costly. Going by own vehicle is often more lucrative.
Rajdhanis are more successful than Shatabdies, as most of them are having an almost full length composition as against only a few shatabdies are near full length rake. Many shatabdies have upto 10 passenger coaches.