It's also bad writing. So how long would it take for the Enterprise to cross through the cloud to get to V'ger itself? And why on Earth would Klingons attack a cloud?
Of course you could try to rationalize that the poor ol' Klingons probably never realized how big the thing was, but really, bad writing is bad writing.
Edit: My math on that may be sketchy. Let me look for an on-line calculator...
Yep, I was wrong, and fixed it.