Eichel simply put is a spoiled brat who is killing Buffalo. He got his coach last year fired and look at the result thus far this year. I take Marner every day every time over Eichel...I also believe Marner is the better player. If Marner was the only young star on the Leafs he would have more points and see more quality ice time.
The opposite could actually be true.. The higher you play up the line-up the greater the Q of C you face and more the opposition plays their best players against in defense including Dmen.
Right now Marner is sheltered by Auston and Willie and plays on the 3rd line. Matthews draws the opposition best nightly and Kadri is tasked with shutting down the opposition best as well as #1 and #2 lines as teams 1 & 2 lines go head to head.
That allows Babcock to shelter Marner on the 3rd line with JVR and Bozak as they play nightly against the oppositions 3rd and 4th lines and 2nd and 3rd D pairings. This creates a gigantic mismatch for the Leafs and allows that trio much easier opposition and opportunity to exploit other teams and statistically they flourish. When Babcock plays that trio against better competition they get eaten up defensively and their +/- numbers soar in the wrong direction. So if Matthews wasn't here and Bozak line was #1 (with Marner) then we have a good idea how bad things might look nightly.
To Babcock Marner is a
3rd line RW and point man on the #1PP unit and its putting Mitch is a position to excel offensively in both a sheltered role, favourable match-ups and prime offensive opportunities as well as man up on the PP with lots more time and space to operate out at the point. While Eichel is in the Matthews role playing against the oppositions best and expected to carry the team with a much tougher of Q of C as a result.
Babcock also doesn't want to play Marner with Matthews because it lets the opposition have an easier assignment in shutting them down and as such wants them each driving their own lines and making them tougher to check.