The Shastras prescribe only a half pradkashina for Shiva.That is why we do not complete the pradakshina.
For the Pancha Devata Deities the rules(for pradakshina ) are as follows:
Eka Chandaya Raveh Sapta Tirstra Karya Vinayake||
Hareschatarstrah Kartabyah Shivasya ardha Pradkshina||
(For Durga(Chandi) (one should do) 1 pradkshina,for Surya(Ravi)
7,for Ganesha(Vinayaka) 3,for Vishnu(Hari) 4 & for Shiva only 1/2.
Refer to image above.The pradakshina should not go beyond the "Gomukhi".As the holy water constantly drips and gets collected there(this is the most rational reason i can think of).
By half pradakshina is meant a "SomaSutra Pradakshina" as stated in the next Sloka:
Shivam PradaksiniKurvan Somasutram na Langhayeth,Iti Vachanantaraat.
(While doing Shiva Pradakshina the order(of Shastras) is that one
should not break the Somasutra).
By Somasutra is meant the Half Moon like curve that is traced out in the process.