The two lines of crit on Relequen outperform the two lines of Weapon Damage on Yandir, and although Relequen’s line of Max Stamina is weak, it still gives more benefit than Physical Penetration if you are at the pen cap in an organized group. These stats alone can make up a 1.5 to 2% DPS difference before considering the main 5 piece bonus.

Relequen’s proc is typically around 8% of single target DPS. With absolutely perfect uptime on Yandir, the Giant’s Might and Endurance buffs can approach that same 8% DPS, but this also requires performing heavy attacks every 20-25s, which are a DPS loss in any rotation. Relequen can be kept up 100% on a lot of bosses while performing a full light attack rotation, and not keeping track of buffs, timers or when to heavy attack.

So basically Relequen is just easier to use, has better small stat bonuses, and overall gives better single target DPS.