Could be the split of our "top-4" defensemen. We have Johnson with Prout at even strength, Tyutin with Savard, Wisniewski with Murray, and then Nikitin is a nomad depending on who is scratched.
If we instead ran:
Johnson - Wisniewski
Tyutin - Nikitin
Murray - Savard
Prout
Then I think we would see TOI much more concentrated in our top-4. Instead, I think Richards wants to more evenly distribute ice time amongst the pairings, bring an offensive and a defensive game to all 3 pairings (not have one pairing be all offense and one be all defense), and chaperone the younger guys (despite them obviously having much of Richards' trust and being a year older).
One more thing is injuries. As guys, particularly young ones, get more NHL game experience their TOI should go up gradually provided Richards likes them. Last year, with a lot of injuries, we saw many more defensemen at the NHL level. That means a lot more "1st, 2nd, 3rd NHL game of the season, haven't seen NHL action ever or since last year" scenarios where Richards has to ease them in and rely on the available veterans more. If you have a bunch of injuries, you are constantly seeing new faces and shuffling pairings which makes most coaches more conservative with ice time and they look to lean on a constant (that being the veterans who are healthy).