What is it?
At least 2 of the main characters either live alone or might as well because of a few reasons:
1. Parents are dead and some how left their high school aged child with enough money to live on.
2. Their parents or siblings are engulfed in work/school so much that either they're NEVER home or ALWAYS away for business.
Worst Offenders?
Basically any type of Harem or romantic type anymore it seems...there's always that one person that lives alone.
Quick example that just popped into my head:
Here
Kaede Sakura (Kampfer)
What makes it popular?
....I guess a lot of people wished when they were younger that living alone or damn near alone without having to pay anything or next to nothing is awesome? Also, you need to have a place to get it on I guess
Reasons I hate it?
I went all through High School and a year through College living with my family and with ALL the friends that I had minus the ones that were in student housing in college. There were NONE that went to the lengths that manga/anime does with, "Oh my parents are really important business people so you might see them once throughout the whole series and you'll be scared shit-less of them."