I'm really surprised that nobody was able to answer this. Here are the reasons:
1973 was the first year of the final 5 in the SANFL
They played all finals on a Saturday at that time, so they needed a second ground in order to play 2 finals on the same weekend.
As Football Park wasn't ready, they elected for Norwood Oval, being the more central location.
The Elimination and 1st Semi Finals were the lesser of the two games in status, Adelaide Oval was the favoured football venue at that time, so those games were played at Norwood Oval. This was announced prior to the start of the season.
Yes, all these reasons WERE given by the SANFL when the announcement was made.
It had nothing to do with giving a team an advantage. The season earlier, Norwood made the finals for the first time since 1965, so they were no certainties to make the finals in 1973.
Interesting to note someone suggesting the final be moved from Norwood Oval because Norwood were in it. We would have looked a bit silly trying to move the 1964 GF from Adelaide Oval because it was South Adelaide's home ground.
At the time of the match, history reported that Norwood would have been better off playing Port Adelaide at Adelaide Oval, rather than at Norwood. To that time, Norwood and Port had played 56 times at Norwood Oval, Norwood won 28, Port 27, and one draw. At Adelaide, the two sides had clashed 25 times, Norwood winning 15, Port 9, and one draw.
No such thing as a home ground advantage in a final. Norwood defeated Port in the EF because they were the better team, and North defeated the Redlegs in the 1st semi for much the same reason.