DISCLAIMER: This is my own personal opinion
Tarang TV has dominated this position because it steals show ideas from other channels and airs them on its channel.
Example in point - The plots of the serials Tarini Akhira Tara and Asha have been copied from two of Disney Star Network's popular shows. Tarini was copied from Ek Hazaron Mein Meri Behna Hai and Asha was copied from Sreemoyee. The latter (Asha) is the only reason why Star Kiran's Shanti based on Sreemoyee failed and was replaced with Mo Jeebana Saathi (a remake of DABH), While Star Kiran's other remake shows continue to be popular regardless of TRPs.
In my opinion, it is blatant malpractice being carried out by the channel to stay on top of the TRP charts. And no one is out there to question it. Recently, they have launched another show which is a copy of Gemini TV's Pournami. All these adaptations are unauthorized, and the other competitors are not doing anything to stop this. So this scenario will remain. Zee Sarthak of course has introduced some recent shows, which are very different from its regular lineups such as Mu Bi Bijoyinee and Khushira Chhunka. So that may be the reason why it is dominating urban charts.