See, if Wakefield is one of the first guys you bring up, there's your problem
wakefield and dickey are the ones that did it quite successfully recently. i guess its viewed as a novelty, but for guys that arent that gifted, but have the ability, it would make sense. seems like some people would want to focus on it to separate themselves from the pack if its clear they struggle to get traditional outs.
Wakefield had 3 good years--1 as a reliever--and was tremendous in one start on a huge stage (03 ALCS vs. Yankees....edit, I also see that he was good in the playoffs his rookie season for the Pirates) but otherwise was a .500 record, 4.5-5 ERA, 1.4 WHIP guy who didn't pitch well in the playoffs. Solid career....but when that's the "patron saint" of modern knuckleball pitchers, that aint good.
Really, Dickey has put up better numbers than Wakefield but is more maligned because he signed that big contract after his Cy Young season.
Someone above mentioned consistency, and I think that's the big thing. Go look at Wakefield and Dickey's numbers. I may be wrong on this but that seems like a hell of a lot of variability from season-to-season--more than your typical MLB pitcher. Wakefield had an outstanding 1995 (2.96 ERA) and was up to 5.14 the next year. He was above 5.00 for most of the next several years and then was 3.90 - 2.81 - 4.08 - 4.87 - 4.15. Dickey has been a little more consistent in terms of ERA but you do see a lot of fluctuation in WHIP, strikeouts, and walks. He went from being a 5.5 K/9 guy to 9 K/9 his CY season and then was back down to 7 the next couple years and then 5.3 the next year and then 6.5 the next couple
Plus, it's really hard for catchers.
Intuitively, it makes a lot of sense that more of your standard 3-4 pitch, 90-93 MPH starter who can't get past AA looks and says, "Maybe I should learn the knuckleball." But it is really, really hard to do and there are few that can teach it. And it's not a pitch that you can just learn to throw to compliment your other stuff.