Since Saturday Night Live is planned days in advance, they typical do not have enough time to report on something that happened a few hours before broadcast. This was the rare exception. The flub made during the Republican debate on ABC happened about 4 hours before Weekend Update aired live.
In the 1970’s when SNL first started broadcasting this bit about the debate would probably never would have happened. Writers and producers for SNL would not have had easy access to this footage from a rival network, let alone know what was so funny. In today’s age, people are instantly notified when something happens. Plus, it was easy for NBC to acquire the ABC footage because they could easily download it off of a DVR or online video. NBC was able to use the footage because they were actually reporting about it in a journalistic sense, and in a parody sense, both of which would qualify for fair use.