Why is Sooryavansham overly broadcast on Set Max / Sony Max?

Edited the below answer because I found a more credible source from mint, one of the more credible business news media

Because it is still popular with the viewers. I found one source quoting the BARC (Broadcast Audience Research Council) stats here

Sony Max was ranked fourth in week 4 of 2018 with 639.675 million weekly impressions. The channel also topped BARC’s top five Hindi Movies channels weekly list in HSM (U+R) and HSM (U). The sequel of SS Rajamouli’s magnum opus – Bahubali 2: The Conclusion was among the five most watched movies in HSM (U+R), HSM (U) and HSM (R). Hindi feature film (HFF) Sooryavansham, that was aired on the channel in week 4 was amongst the five most watched programmes in HSM (U+R) and HSM (U). Another HFF that was aired on the channel, Bhairava, was also among the top-five Hindi movies in HSM (U).

HSM - Hindi Speaking Market. U - Urban. R - Rural.

From another source confirming the same results

Baahubali that premiered in October last year and Golmaal that was aired in February this year, both had one common name that made it to the BARC list of top five films that week—Amitabh Bachchan’s 1999 drama Sooryavansham.

A remake of Tamil film Suryavamsam (1997), the E.V.V. Satyanarayana directed film notched up 4.4 million BARC impressions on Sony Max compared to Baahubali’s 26 million on the same channel and 4.7 million impressions compared to Golmaal Again’s 16 million on Star Gold. Impressions refer to the number of individuals (in thousands) of a target audience who viewed an event, averaged across minutes.

Sony has bought the movie rights for 100 years according to this source.

It has reached a cult status among Indian audiences. According to Neeraj Vyas, senior vice-president and business head, Sony Max cluster, Sony Pictures Network

"Television viewing essentially is community viewing. Television, is a representation of the average Indian, the mass consumer who enjoys stereotypical scenes, situations and songs and comes back to them whenever he can. A family entertainer like Sooryavansham will always work on television because it not just allows for emotional peaks and curves every couple of minutes but can be watched in segments or its entirety with at least one other member of the family multiple times without discomfort."

So as long as viewers are willing to see it, Sony Max will broadcast it.

Dread it. Run from it maybe. But Sooryavansham still arrives on Sony Max.