Short distance may not be the issue. For Eg. Trains running between Bangalore to Mysore (140kms) have tatkal seats, then why not Ahmedabad to Surat (230kms)? Thing is few trains dont have tatkal quota earmarked. For eg. Circar Express (17644) has tatkal seats that can be booked from Kakinada to Madras, where as its pairing train 17643 does not have seats earmarked in tatkal quota. So tatkal booking is not permitted in 17643.