Because Star Trek V sucked? Because there was no redeeming feature to save from that movie?
I think there were ideas from that movie that could've been salvaged and presented in a different context.
But some things, like Kirk's "I need my pain!" speech... they couldn't do that because the characters haven't reached that point yet.
And it was a rhetorical question.
STV has the quality of a good TOS episode. If you say you hate STV for it's silliness, plot holes, camp and visual effects, then you surely must hate TOS to death.
All moments between Kirk, Spock and McCoy are pure gold in this.
The scene in which Sybok shows them their pain, and Kirk's "I need my pain" is one of the greatest scenes in the Trek films.
I have a soft spot for it, especially that scene.
But one could ask the same question about any remake: why remake so many good movies when there are plenty of bad ones (or at least seriously flawed ones) that could be remade?
And again, I was simply thinking out loud.