According to the following link, Amherst College and Smith College played every season from the 1977-78 season through the 2006-07 season. They didn't play again until last season's NCAA Tournament.

https://gosmithbears.com/sports/womens-basketball/opponent-history/amherst-college/20

This season, Smith scheduled 5 NESCAC colleges in their schedule. In 2023-24 and 2024-25, Smith played 4 NESCAC colleges in the regular season.

Does anyone know why Amherst and Smith College do not play annually in the regular season?

The two colleges are separated by 8 miles and have both gone to multiple Final 4s in recent years.  It seems like it would be a great Division 3 non-conference game. I wonder if one school is ducking the other in this situation.