It could be a number of different reasons...
First, Brevin has missed a chunk of the season, so they weren't gonna rush him back & immediately start playing him heavy reps off that bat, he was on a pitch count & that's smart because you don't want Brevin reaggravating his injury.
Another main reason, is that it's based on the type of Defensive fronts & looks we get, Lashlee sees how Defenses play us & feels as though we have a better advantage by running primarily 11 & 10 personnel. Also have to remember we go fast tempo, which means we run a lot of plays & it's easier to get lined up & go fast in different personnel groupings, if you run a base of 12 you run the risk of having to sub both your guys out if they get tired & for us that can be detrimental because we need a TE to block for us in a shît ton of plays.
You have to also factor in that we don't have a consistent stout interior OL, so in the instances in which we do run 12 P that 2nd TE will almost always be used as an additional blocker.
I understand the gripe, I would love to see us run 21, 22 & even 23 personnel, but yall have to understand we didn't have a Spring to implement a full offesnive install & Fall camp was truncated, we don't have the OL to run the wide variety of formations we'd all like to see, Lash has gone with what works & what we can execute the best.
Regardless of all the bellyaching, our Offense is distinctly better this year than last & we can score, of course it's not perfect but it's almost as if you guys have no idea that our OL is bad & considering the Covid season you would think yall would realize we're not operating on a normal basis & schedule.
Considering the circumstances the offense moves the ball well & a can score frequently when it matters most, give a Lash a full offseason & a better OL & you'll see a lot more variations.