How did Rod Steiger win Best Actor when

Back then Steiger was considered one of the foremost actors in America and it would have been unthinkable for him to have been put forward for supporting for such a large and dominant role. It was a two-lead film - it was only in the last twenty-thirty years that Oscar campaigning has started consistently pushing co-leads as supporting to get easier nominations.

If something like, say, Carol or Once Upon a Time in Hollywood had been released in the 60s-80s, Rooney Mara and Brad Pitt would have been rightly considered co-leads - and, if In the Heat of the Night was a 2019 film, Steiger would have been shoved into supporting.

I do agree that Newman was better, but that year's lead actor line-up was a notably strong one - so strong that Poitier was boxed out. (I'd probably have gone with Hoffman of the nominees, although my favourite of the year - Robert Blake in In Cold Blood - was overlooked).

I wouldn’t say Poitier was “clearly” the lead actor. They seemed about equal. Poitier’s character appeared more as the hero because he was braver, smarter. But in terms of plot and screen time, they were equals.

The same reason you danced around the real subject of your complaint instead of telling it like it is.
Exposure.
Proving how irrelevant, commercialized, and vain to the point of vulgar the whole proceedings are.

poitier had 2 movies at the time

Your post reminds me that Anthony Hopkins got LEAD actor also for The Silence Of The Lambs even though it was clearly Jodie Foster's movie and his on screen time I believe was roughly 15 minutes.

Poitier and Steiger were co-lead actors.