Why was South Korea able to implement democratic reform yet the north wasn't?

Perhaps the question should be asked the other way around: Why has North Korea been able to avoid democratic reform, while democratic reform was forced on the dictatorship in South Korea?

This is fairly straightforward: the government in the North has a strong grip on power. Overt dissent is crushed promptly, and there is no opposition leadership in the country. There is no opposition movement in the country. There are opposition movements in the North Korean defector/refugee community, perhaps most prominently the North Korean People's Liberation Front, based in South Korea. Communication between such foreign-based opposition movements and the North Korean people is limited, and these opposition movements cannot provide effective leadership for opposition within North Korea.

Since North Korea is somewhat of an international pariah, and is relatively insensitive to threats and cajoling by the West, international pressure is unlikely to lead to democratic reform in the near future.

There have been protest movements in North Korea. One issue important to many has been the continuation of private trade (something the government has sought to ban in an excess of communist fervour). The government, at least de facto, gave in, and continued to allow private trade. The North Korean government is sensitive to popular anger, and pragmatically makes concessions. A mix of the prospect of brutal repression and sensible concessions by the government have so far kept peace.

In contrast, South Korea was nominally democratic even during the dictatorship, and, at least to keep up appearances, allowed some opposition movements. Since they depended on trade with the international community, and military support from the US, they could not completely ignore international opinion and US opinion. While the South Korean dictatorship imprisoned, killed, and tortured various opposition leaders, and suppressed opposition movements, they were not in a position to transition to a bare-naked dictatorship without the thin veneer of democracy that they maintained. The US was willing to go along with violent and brutal suppression of the pro-democracy movement in the Gwangju Massacre (but was perhaps unhappy with the extreme and brutal response), but when the next major round of protest came, in 1987 in response to the upcoming un-democratic election, the US made it clear that they would not support a repetition of Gwangju (sine the 1987 protests were much bigger and more widespread, it would have been Gwangju times 10, in the streets of Seoul). The government ordered the army into the streets to suppress the protests, and the US ambassador had strong words with them, and the orders were countermanded before the army actually deployed. Reagan, more gently, urged the government to accept a transition to democracy.

In contrast, North Korea does not depend on the goodwill of the US, and doesn't depend on US military support. While the US might protest violent suppression of a pro-democracy uprising in North Korea, there is little other than protest that they could do.