I think that turned out to be for the better as it made the mnemonics far more consistent and enabled the more complex addressing modes to be specified easily.
The only remaining inconsistency was "JP (HL)" (really should be "JP HL"), which Intel fixed with the 8086 when it adopted its variant of the Z80 syntax; the fact that both "JMP BX" and "JMP [BX]" are available on x86, and do different things, might've motivated this.
(Oddly enough, this inconsistency still exists in GNU/AT&T syntax.)
The only remaining inconsistency was "JP (HL)" (really should be "JP HL"), which Intel fixed with the 8086 when it adopted its variant of the Z80 syntax; the fact that both "JMP BX" and "JMP [BX]" are available on x86, and do different things, might've motivated this.
(Oddly enough, this inconsistency still exists in GNU/AT&T syntax.)