Here are my thoughts on potentially why Allen and the other TE's weren't targeted as much as we expected in 2021.
1) Tight ends are most effective in play action passes, but PAP's require longer protection from the offensive line, and we know that was a weak point in 2021.
2) Going into the season NU was going to use a lot of 2 Tight End sets with Allen being used all over the field to exploit mismatches. However, with Vokalek's injury in the first half of the season, that limited the 2 tight end sets which could be used and Allen had to be used on more traditional TE roles.
3) Passing to the tight end over the middle can sometimes require the pass to be fit into tight windows within coverage. We know that wasn't Martinez's strong suit, and AM was consistently told to limit the turnovers. So, AM might have had an initial read with Allen, but gone off him if Allen wasn't wide open.
4) Teams may have been sliding coverages to Allen early in the season, to force unknown NU receivers to make plays. Toure might have been a big beneficiary of more attention paid to Allen, and was able to find a lot of open spots deep, behind the coverage going to Allen.