Warlock is popular because it is such a fun class to play, and that has a lot to do with Kalo's fantastic and clean design for the class.
However, Warlock as a theme is uninspiring and one-dimensional. The characters themselves can be nuanced, sure, but the class itself? Not so much. One of the indicators is that Warlocks do not have a function in the immediate societies established in Forgotten Realms. (maybe to some niche cult or small town, sure)
An exercise to recognize what classes are conceptually rich is when we speak the names of these classes. What images do they evoke in your mind?
Wizard: I can imagine all sorts of backgrounds - scholars researching in their high towers, necromancers being creepy in the graveyard, war mages in armor and casting spells in a war, magocracy, and politics.
Rogue: dungeoneer, brute, thief, assassin, tinkerer, smuggler, scout, city guide. There are so many tropes Rogues can fulfill, be it lawful, unscrupulous, or anywhere in between.
Cleric: a local village pastor, a cathedral bishop, a wandering missionary, a secluded temple caregiver - the priesthood and clergy are essential in a fantasy setting where deities are abundant and have manifestations both literal and metaphysical.
These iconic classes can inspire roleplay by just their names alone.
Warlocks? Well, they sacrifice something to gain personal power from some entity surrounded by mystery, right? Or perhaps they need to perform some ritual from time to time? Sacrifice something in the name of their patron? So what is their function in conjunction with the rest of the society? What is their interaction with the rest of the society with what they do?
Wizards, as mechanically poor and irrelevant as they are now, remain the 2nd most popular caster class, right behind clerics. Warlocks, which I consider the opposite case, where their mechanics shine (thanks to Kalo, again) but are conceptually uninspiring. The two W's make for a fun case study.
To wrap this all up and get back to the point, I don't blame people for not having a good time interacting with Warlocks because the theme and concept weren't that clear-cut or great for social interaction. It is not on the developer to fix whatever problem is perceived with Warlocks, but that responsibility lies with the players and the player base. Maybe with some nudges and pushes from the DMs and the lore writers, players can make Warlock RP interaction a more enjoyable process for everyone involved.