It took so long because as the other commenter said, it's a huge project that required many months of preparation and additional construction. The reason they had press crews recording as they switched is because this is the culmination of a lot of very careful work. If they hadn't gone slow and made sure everything was perfectly ready and followed the required steps exactly as they needed to be done, in the right order, and at the right time, then you would have been without power for literally weeks or months as they repaired the damage to the power grid and got all connected power plants repaired and started again. Which also requires power, which they would have had to figure out how to provide, since you can't just start power plants cold. You might even have needed to repair your own electrical wiring in your home, if they fucked up and blew shit up.

You say fair enough, but I still don't get the feeling you understand exactly the enormity of what was just accomplished, and the engineering that went into it. Believe me when I say that, at the time they switched over, there was a lot of people sweating very hard and holding their breaths, praying that they didn't make any mistakes.