I can give you some idea why Airtel is finding it hard to enter some areas.
While Jio manage their expansion and infrastructure by their own people, Airtel is dependent on LCOs (local cable operators) for their last mile connectivity.
Though this article is old, but it should give you idea what I am talking about:
Airtel to expand home broadband through LCO model over the next 6 to 12 months: Gopal Vittal
They are doing this since year 2020 to avoid cable cutting and other problems given by LCOs. However, some areas still have LCOs who either doesn't want to pay the security amount that Airtel needs for materials and other stuff or don't want to work as per Airtel's agreement. So Airtel can not enter those areas.
I can give you example of my city. My city has 30% Airtel Xstream Fiber coverage since quite some time. The rest areas, including my area, falls under different LCO whom Airtel was approaching since 2020 for tie up and he was hesitant. Now after AirFiber launched, he finally agreed to allow Airtel Fiber expansion in our area because Airtel started installing AirFiber in our locality which he couldn't stop. And this is not only my area, in many cities LCOs are now agreeing to allow Airtel tie up with them because of AirFiber entry.
While Airtel only give some commission share to the LCOs for the last mile connectivity and installations, Airtel themselves manage their fiber network similar to Jio. Airtel also keeps the OLT (optical line terminal) access with themselves, it is placed near the towers similar to Jio Fiber OLT and those things are configured by their own engineers. Due to this, the network up time and stability remains top notch.