Yes, it is absolutely ok to have Reconciliation on Sundays, too (or Monday, or Tuesday, or Wednesday, etc)! My guess would be that logistics just make the scheduling more difficult on Sunday. The Roman parish where I grew up, for instance, had
Saturday 5:15 Mass
Sunday 8:00, 9:30, and 11:00 Masses.
Consider that people might linger, converse, want to speak briefly with the priest between Masses, introduce a new baby or a visiting relative, etc, it would have been difficult to schedule a decent formal length of time for Sunday Confessions.
By contrast, every Maronite parish I’ve ever attended offers Reconciliation on request 30 minutes before any Liturgy, so as to provide broader availability. There is no distinction made between Saturday or Sunday for this purpose. You just approach the priest and say, “Father, could you hear my confession?”
Hope that helps!