why is nobody picking deadlock? is she that trash
You will have to play another sentinel for flanks. So, if you play her, you will have to give up another slot on the team on most maps.
Playing her needs the whole team to change their playstyle. She is only good for stalling executes, denying plants, and denying defuses. However, to actually deny plants and defuses you need another agent with some kind of damage ability. She can only get you the info of the defuser/planter. So your whole team comp has to be planned around playing Deadlock.
Deadlock does have the potential for some crazy combos with other agents. I do think she will be good if picked as a wildcard because she will help secure kills of planters and defusers who are currently able to move away when the util is thrown. The free kills for 3-4 rounds in a game could help, but IDK if it's enough to give up on other useful agents.