Why were the stock of Bharti Airtel and Bharti Infratel so weak on 24 April when Infratel had announced a good set of quarterly numbers?

You are right that the numbers of Bharti Infratel were quite encouraging. In fact, Bharti Infratel reported net profits of Rs.650 crore for March 2020 quarter compared to just Rs.608 crore in the year ago period. Even, consolidated revenues were marginally higher for the quarter at Rs.3,624 crore. However, we may have to take these numbers with a pinch of salt as Bharti Infratel had adopted a new accounting system from April 1, 2019. What really spooked the stocks of Bharti Infratel and Bharti Airtel was that these companies decided to extend the deadline for merger with Indus Towers to June 24 considering the short term pressures created by the COVID-19 pandemic. The company had a total 95,372 mobile towers, which included 42% of the units managed by Indus Towers.