Reason #2: Los Angeles Kings didn't want to change all their defensive pairs
Another possible reason is that the coaching staff didn't want to change all their defensive pairings with the return of Drew Doughty. Prior to his return, it was reported that Doughty would be paired with Jacob Moverare and that is who Clarke had recently played with.
After the Penguins game, the Kings shuffled their lines, and Clarke, who had been playing with Joel Edmundson, was moved to play with Moverare and Spence replaced him alongside Edmundson. If the Kings moved forward with their plan of Doughty playing with Moverare then Clarke could have returned to his pair with Edmundson but truthfully Spence has been better than him.
This would then mean Clarke is the odd man out but the Kings have spent most of the season with seven defensemen active for games. There isn't really much of a reason not to return to this and have Clarke available for games, especially on power plays where he has two power play goals and five power play assists.