The real question is, what viable targets does Psystrike (or Psycho Cut or Zen Headbutt) have that Low Kick, Earthquake and Stone Edge don't cover almost as well or better?
Despite being STAB, an uninvested Psystrike is barely more powerful than Earthquake/Stone Edge on neutral targets:
4 SpA Mega Mewtwo X Psystrike vs. 4 HP / 0 Def Palkia: 157-186 (48.7 - 57.7%) -- 95.7% chance to 2HKO
252 Atk Mega Mewtwo X Earthquake vs. 4 HP / 0 Def Palkia: 146-172 (45.3 - 53.4%) -- 35.5% chance to 2HKO
Note that this also forces Mewtwo to use a nature which compromises one of its defenses somewhat; if it opts for a Jolly nature instead, Psystrike is no more powerful than any of Mewtwo's other moves on neutral targets, and Low Kick, Earthquake and Stone Edge have perfect neutral coverage.
So the only targets that remain for Psystrike are Pokemon which are hit super effectively by Psychic-type moves, but not Fighting-, Rock- or Ground-type moves. In the context of Pokemon that are actually viable in the tier, this means only two targets: Gengar (before its Mega Evolution) and Arceus-Fighting. Yet even for these targets:
4 SpA Mega Mewtwo X Psystrike vs. 252 HP / 0 Def Arceus-Fighting: 270-320 (60.8 - 72%) -- guaranteed 2HKO
252 Atk Mega Mewtwo X Low Kick (120 BP) vs. 252 HP / 0 Def Arceus-Fighting: 223-264 (50.2 - 59.4%) -- guaranteed 2HKO
252 Atk Mega Mewtwo X Stone Edge vs. 4 HP / 0 Def Gengar: 220-259 (83.9 - 98.8%) -- guaranteed 2HKO
With or without Psystrike, Arceus-Fighting is consistently 2HKOed, and Psystrike would only be advantageous in rare situations (not to mention that most Arceus-Fighting cannot significantly harm Mewtwo if it carries Taunt). As for Gengar, to survive against Mewtwo it has to refrain from Mega Evolving, which means that Mewtwo is free to switch out. Even if it does not, with only little prior damage, Stone Edge OHKOs Gengar.
Therefore, the coverage provided by Psystrike is rarely ever useful. Even if it can be used to bypass burns, Reflect or Intimidate, it really is not worth a moveslot. If you are insistent on using a Psychic STAB move, Zen Headbutt at least is noticeably more powerful than Earthquake/Stone Edge against neutral targets, as well as having a 20% chance to flinch, but even that rarely compensates the loss of a coverage move or Taunt.