Because even though he was arguably the best player in the National League briefly (in 1978), his career totals are 339 HR, 1,493 RBI, and 1,272 Runs—none of them hugely impressive totals for someone who played 19 seasons. He was only a “very good” player for about seven years (‘75-‘79 and again in ‘85-‘86), which means that for 12 years of his career (two at the start, five in the middle, and five at the end) he was a pretty mediocre quantity. And Parker was really only a great player for one season. In fact, it’s much the same argument that applies to Jim Rice also, great briefly in 1978.
While Parker does compare favorably to Al Kaline, Chuck Klein, Rocky Colavito and Paul Waner, he’s still a notch below Dwight Evans and Bobby Bonds. Also, Gary Sheffield, Vladimir Guerrero and Sammy Sosa have a much more impressive block of seasons than Parker does. The question then becomes whether Kaline, Klein and Waner are above or below the imaginary line, since Parker fits in that group.
Marks against him:
He was not the premier right fielder in baseball for any block of seasons. During his career, this would instead go to Reggie Jackson 1972-80, and Tony Gwynn 1982-87. (In contrast, Jim Rice was the best left fielder in baseball for the period of 1977-79).
His last four seasons were as a DH, and a DH posting OPS+ numbers of 103, 110, 118, 81 is basically a washout.
In the five seasons from age 29-33, he didn’t reach 20 HR any of those years.
Topped 60 BB in a season only once.
Only a 58% stolen base success rate.
Going for him:
Five top-5 MVP finishes, which is very impressive.
Four top-5 OPS+ finishes.
Nine top-10 RBI finishes.
3 Gold Gloves.
Personally, I think he deserves to be in, just barely, but only because I believe a lot of right fielders are deserving, including Sheffield, Evans, Bb. Bonds, Guerrero, and Colavito as well, who are at or above Parker.