Why do we say on television but say on the radio? I saw these two mentioned in Cambridge dictionary entry.
We often hear and use it both in private and in public settings and in films, on television and on the radio.
I think in the early radio days, the focus from the listener's perspective was on the radio device, and that there was little technical knowledge about how radio worked. So "the radio (device)".
In the early days of television this was probably the case too. But over time, the entire media spectrum known as "television" has become widely known. Not just the television set, but also the creation, production, distribution, transmission, etc. that constitutes the entire system.
Not referring to a physical device but rather to a business and economic segment, it becomes uncountable so no the.
Also, if you consider that "T.V." is commonly used in place of "television" nowadays, "What is on T.V. tonight" is preferable to "What is on the T.V. tonight".
But in the final analysis usage (or not) of the article is more likely influenced by locale and personal experience, and I doubt any hard rule exists.