why are using ankle weights when doing lyk plyo or when playing bball bad and why would the gains go away?

__________________
"Give me the ball and ill carry the team"
stats:

Age: 15
Height: 6 foot
Weight: 163lb
Running vert: 30
Squat: 148 (326lb)
Deadlift: 138kg (304lb)

Novice weights stats

Starting:
Squat: 78kg (171lb)
Deadlift: 88kg (194lb)

End of Phase 1:
Squat: 98kg (216lb)
Deadlift: 105.5kg (231lb)

End of Phase 2:
Squat: 128kg (282lb)
Deadlift: 123kg (271lb)

End of Phase 3:
Squat: 148kg (326lb)
Deadlift: 138 (304lb)