Edgar hit for a better average (by 39 points), walked a lot more (39 points of isolated patience), and hit for a lot less power (101 points of ISO). OBP is more valuable than SLG and batting average matters, which is why Edgar's OPS, OPS+, RBat, and any other attempt to quantify total hitting quality has Edgar higher. On top of that, Edgar had almost twice the RE24 and WPA and 2 wins ahead on Clutch, so one couldn't make that argument.
https://stathead.com/tiny/y47Lr
Belle had an all-time great slugging season. Of the 14 players who have recorded 100 extra base hits or more in one season, Belle had the lowest OBP:
https://stathead.com/tiny/y47Lr
Of the 25 players who have achieved a .350 ISO in a qualified season, Belle had the 2nd lowest OBP (ahead of 1997 McGwire). It's also the 6th lowest SLG. That's because his batting average was relatively low compared to those guys (8th lowest) and he had the 2nd fewest walks out of any of them (and the lowest walk rate, as only 1994 Bagwell is behind him). 50 HR and 50 2B is a very cool combo, but I'd rather have Gehrig's 18 triples with his 47 HR and 52 2B (1927), or Bonds's 73 HR and 32 2B in 70 fewer AB (because no one pitched to him, 2001). Compared to all time great hitting seasons, his average was low, he didn't walk a lot, and too many of his XBH didn't clear the fence.
https://stathead.com/tiny/Xj5P9
All of that is why his 177 OPS+ was only third in his league (behind Edgar & Frank Thomas) and is around the
213th best batting season of all time , as opposed to Edgar who was around 138th. That's 181st & 122nd if you limit it to 1900 and later, 168th & 108th if you then require 502 PA, and 96th & 57th if you then limit it to 1947 and later. Having one of the 100 best batting seasons since integration is extremely impressive! But Edgar was a better batter.