That’s more based on the simple fact that no one in World of Warcraft is PVPing period. Its not a warlock specific issue or viability issue. The last stats were like less then 9% of the player base have competed in a single rated arena of any bracket.
Pops dwindling due to poor choices by blizz and hopefully that pickup happens a bit in 9.1.5 but you not finding a grps not a lock issue anymore then a pop issue.
If we wanna say there’s 10 pvpers obviously they won’t pick lock blah blah… at that point they look at comp viability with classes available and splay with all 3 specs is a strong comp in overall meta.