We know that the First Evil and its Bringers are sufficiently well-organized to infiltrate and destroy the Watcher's Council. So, what prevented them from doing the same to Buffy's house with her team and all the Potentials inside?
The answer I often come back to with questions like this, (i.e. "why didn't the bad guy do this...?"), is that the hero would have figured it out. That's the nature of the show, both in-universe and out-of-universe.
There would have been an episode about it. Bringers plant a bomb in Buffy's basement, in an attempt to kill her and the whole team. Maybe they would have lured Buffy away so she didn't stop them, but inevitably she would have figured it out, run home, and stopped it at the last second.
Buffy had a well-established habit of saving the day by that point, which the First knew all too well. To kill a bunch of old British people you can easily use a bomb, but to kill a Slayer you need something more complex and nuanced. There are plenty of simple plans that COULD have killed the Slayer, or any other hero, but we have to remember that the villain is watching the same show we are, and the good guy has a bad habit of winning in the end.
So basically, the First didn't try because it didn't think a plan like that would work.
an in universe explanation would be that every single freaking Buffy villain is a psychotic megalomaniac with delusions of grandeur all of them want to win yes but they want to win with style they dont want to just kill Buffy and company they want to destroy their will to fight break their spirit and then kill them