In the past few threads, I've seen people discuss whether warlocks should use int or cha for casting. It is well-known that in the 5e playtests warlocks used intelligence, but complaints from filthy 3aboos pushed the designers to make it charisma instead. However, I submit that neither is an appropriate casting stat.
Intelligence is the casting stat used by wizards, as well as eldritch knights and arcane tricksters. In each of these cases, the character's spellcasting ability is gained through careful study and understanding of magical processes. While some warlocks do delve into tomes to find their patron, this itself does not give them their arcane abilities, and there are plenty of warlocks who meet their patron some other way.
Charisma, on the other hand, is used by paladins, sorcerers, and bards. For each of these classes, magic is an expression of the self, whether that be a sorcerer's heritage, a paladin's willpower, or a bard's emotional expression. Supporters of the chalock argue that charisma is necessary for a warlock to bargain with his patron, but, again, this is not the source of the warlock's power, only the method of first catching the patron's attention. And, again, not all warlocks find their patrons the same way; it is explicitly stated in the player's handbook that some warlocks don't even realize what is happening when they make their pact.
Indeed, it has become clear that a warlock's power comes not from arcane study or the power of the self, but from a deal made with a powerful being. What other class gets its power from such an entity? The cleric, of course. Similarly, both druids and rangers get their power from nature, which, while not exactly an entity, is certainly a powerful outside source. And what casting stat do clerics, druids, and rangers use? Wisdom.
Thus, it is quite obvious that wisdom should be the casting stat for warlocks. Pic unrelated.