I can only conceive of the most unlikely scenarios that result in Brian Williams choosing to wear this tie. For example:
NBC Studios dressing room area has been infested with ants. Exterminators arrived at 4 PM; set off several toxic bug bombs without prior notice. Brian Williams arrives sans cravate, fully expecting to wear the one already in his dressing room. A brand new intern from Pocatello, Idaho is dispatched to "go and find a tie, any tie, in five minutes!" The intern runs out onto the street where he finds a vendor hawking cheap ties; he buys a hideous electric blue one with narrow carnation pink stripes for only ten dollars. He zooms back into 30 Rock, where Brian has no choice but to expertly knot it up and take his place behind the news desk. The floor director signals, then counts him down, "...and in three, two, one!"
This has to be the reason.