Because Clay Matthews hasn't been a great fantasy asset for the most part. He's a big name everyone recognizes, that doesn't mean he's a great fantasy player. Matthews is more important to his team than he is to a fantasy football roster. A couple reasons for that:
1) position- Matthews is an outside linebacker. Generally speaking outside linebackers do not produce as much from a fantasy aspect as a middle linebacker, those guys are the ones you target in your fantasy draft. They have more tackle opportunities and rack up more points.
2) low tackle numbers Matthews just hasn't done enough to warrant a high draft pick with these low tackle numbers. His sack numbers are above average, yes, but he has only surpassed 50 tackles once in his career. In fantasy football, if you aren't putting up 70-80 tackles you won't be drafted as more than bench depth.
3) turnovers- The other major stat category of concern. Matthews has not really created many turnovers in his career... 8 forced fumbles, 3 recoveries, and 4 interceptions.
On top of that, he had some health issues last season. People tend to approach that cautiously.
All in all, Clay Matthews has done little in my eyes to earn a spot anywhere on my roster. Sure the announcers call his name, praise him for the work he does on the field. He gets in the backfield and wreaks havoc on opposing QBs. But beyond the sack numbers he has been a disappointing player in terms of fantasy, one I'd only ever take in a bye week scenario unless there's a dramatic turn around this season. He is a leader on the field and a crowd favorite, but your IDP studs are the guys that get dirty and finish plays.
(sig by soty)