script: Why should CHECKLOCKTIMEVERIFY and CHECKSEQUENCEVERIFY opcode be successful with DROP Opcode?

This is because CSV and CLTV are NOP operation codes that were redefined in a softfork.

Because Softfork can only change the validity of transactions from valid to invalid, the only effect that was allowed to this redefinition is to abort the script in some conditions and, otherwise, continue to act as NOP.

NOP does not take anything out of the stack, and as a result, neither CLTV and CSV can do it. Otherwise, a script such as "0 n CSV 0 EQUAL" would change from invalid to valid.