From NFL.com:
"Player Bio
The third Watt brother decided it was time to join his older siblings in the NFL after an excellent junior season. Following in the footsteps of J.J. (a three-time NFL Defensive Player of the Year) and Derek (sixth-round pick as a fullback by San Diego) won't be easy. In fact, it wasn't easy for Watt to get on the field his first couple of seasons with the Badgers, redshirting as a tight end in 2013 and losing the following season with a right knee injury. Watt then tore up his left knee during spring practice in 2015, having surgery but returning to play as a reserve linebacker in all 13 games (eight tackles, 1.5 for loss). Finally healthy in 2016, he won a starting job and excelled, garnering second-team Associated Press All-American and first-team All-Big Ten honors with 63 tackles, 15.5 for loss, and 11.5 sacks."
There were injury concerns, this is why he didn't have an impact until his junior season