Because of the limits to certain stats

Because of the limits to D2R Items certain stats, and the range of other players as compared to mobs, and the distinctive objectives of Battlegrounds mode The build that worked for the player so effectively in PvE isn't going to perform as well in PvP. 

It's frustrating watching a player's build that obliterated random skeletons and wolves do not make a dent in the enemy Crusader, but it's a an aspect of learning. It is best to design a unique loadout for 

PvP from the bottom up, orienting every single player's skill or gear towards their new goal. A Necromancer who never gave Bone Wall a second thought while killing spiders in the swamp could be inclined to be tempted to look it over while trying to hold back a raging Barbarian.

There's a reason that Demon Hunters don't stand toe-to-toe with Barbarians. Because some classes are more focused on tanking and others focus on DPS (or support), it's logical that when two characters meet on their respective fields, one could have a distinct advantage simply due to the role they play. 

Understanding how the various classes compare to each other is essential to ensure that a weak support player won't be one-shotted by an incredibly powerful DPS strike. Whether the player's ideal position is frontline, in the middle, or on the backline, they must always position themselves so that Diablo 2 Resurrected buy items they can effectively fulfill their task.