With North Korea preparing their own rocket launch it would be embarrassing for the ROK if this launch fails. So it is best to not take any chances and double check everything before launch:

South Korea decided Wednesday to postpone the launch of its space rocket Nuri due to a technical glitch found during final preparations, the science ministry said.
“While controlling a helium valve, aerospace engineers found a communications problem between a launch control computer and a launch pad facility control computer,” Vice Science Minister Oh Tae-seog said in a briefing at the Naro Space Center in Goheung. “The valve is OK, but the system might matter when it goes into the automatic operation system. So we inevitably canceled the scheduled launch.”
The engineers detected the problem at around 3:30 p.m., just before they were to start injecting fuel and oxidizer into the rocket.
Yonhap
You can read more at the link, but ROK officials are still determining if they will be able to launch the rocket with its 8 satellite payload either today or later in the week.