Why doesn't Redd ever come to my island?

Unfortunately random visitors are, in the end, random. There are certain visitors that are guaranteed to show up on certain days, but most are put into a list that is randomly selected from to determine which visitors will appear on your island throughout the week.

On Saturday, K.K. Slider is guaranteed to come. On Sunday, Daisy Mae is guaranteed to be present in the morning. That leaves 5 weekdays where random visitors are pulled from the list. Saharah and Kicks are guaranteed to fill two of these slots, and during certain events, other characters make take other slots (for example, if it is during the Nature Day event, Leif is guaranteed one of the slots). That means in the best case, there are 3 slots for the 7 remaining visitors.

The three visitors that are selected to fill slots during any given week are given reduced priority for the following week, which means they won't be selected. That means if you don't see Redd for an entire week, there is a 3/4 or 75% chance that you will see him in the next week.

However, this still means that each sequential week still has a 25% chance that you will not see a specific character. While this is fairly low, it is still possible, and there is nothing you can do to increase the probability of encountering them. It is likely that you are simply getting unlucky. Make sure you visit your island every single weekday to ensure you don't miss him when he finally shows up.