Why is OP_SUBSTR disabled? (and OP_LEFT, OP_RIGHT)
I'm afraid that my answer here can't be better than "Ask Satoshi".

There was a series of bugs found in Bitcoin's early life, and the response by its creator was to disable a number of scripting opcodes. Some of these may have been bad choices (or very wise ones...), but we don't know, and it does not matter. For all intents and purposes, these opcodes do not exist, as "reenabling" them would be just as hard as introducing them as new operations in the first place (i.e. a hard fork).