3 comments
Maybe it's to God's harsh treatment of Ananias an Sapphira, and Paul's(?) warning to teachers of how they will be punished more harshly for wrong teachings?
One, symbolically Jesus is the rock. Jesus give us living water by first being struck, and second by God's powerful word bringing him back from the dead.
God was commanding Moses to do something to foreshadow the living water come to us in Christ. This is probably why the disobedience is so significant.
Second thought, it seems Moses put his trust in what he had experienced previously, and not in God. His trust was in the power of the stick. God wanted is trust to be in Him.
Third thought, God's word is always good and life giving. To disobey God's word, no matter what the case will result in a form of death.