The easy answer may be that Vada Pinson had many seasons that were very good, but nothing that could be considered great. He racked up many hits (four times he compiled over 200 in a season), but wasn’t always considered a clutch hitter as shown when he batted .091 in his lone World Series in 1961. He had decent speed, but never the led the league in steals. He could field, but only had one Gold Glove. He had a great Batting Average, but barely walked giving him a relatively low On Base Percentage.
Pinson was durable and lasted a long time and socked a lot of hits in his career. He was very good for a long time (far longer than most), but without that one season of being great, the case for Vada Pinson’s Hall of Fame candidacy becomes a tough sell.