I'll give you three reasons.
1. Sometimes the sixth person can be more valuable than some of the starting five. Havlicek is perhaps the most famous example, but UConn has its own precedent in Sales and Ralph, two players as freshmen clearly better than some of the starting five, but came off the bench nonetheless throughout the entire year.
2. One of the factors that gives a sixth person value is versatility, the ability to play multiple positions and/or provide help on defense/offense. That allows the coach to assess what might be going wrong at the start of a particular game and better plug in the ingredient to fix it. In contrast, players who specialize in a role provide a greater chance of starting a game without surprises, something Auriemma in particular values. Muhl is the most specialized, Ducharme the most versatile, out of all the options.
3. Experience also helps to minimize surprises for a coach. Muhl has the most experience of the options, particularly for the needed PG role. She may not get as many minutes in the end if Ducharme really shines, similar to Doty not getting the same minutes as Hartley, but she still is the most likely to start the game off on the right foot, ie, make the least mistakes.
