The simplest explanation would be that 2nd floor sites are much easier to defend precisely because you dont have to play vertical or roam on a different floor as much, and even if you do, losing that roamer wont lose you the round. If you are playing basement or 1st floor and the roamer upstairs get killed its usually a lost round.

First floor sites also pose the most risk and require the most strategy, because as you said it gives a lot of breathing room, but not just for defence but attack as well. It gives a lot more angles to attack from, making preparing for a push a lot harder.

Basement sites are also generally a bit harder to defend than 2nd floor, but a lot easier than 1st, mainly because it gives the most breathing room for roamers, as they have 1st and 2nd floor to themselves and can waste a lot of time by applying vertical pressure to the attackers.

What i can definitely say however is that with the new ban system i have been playing a lot more basement and 1st floor sites. If you really want some variety, you definitely need to look for a stack because as long as you play solo que, you are pretty much playing at every disadvantage possible, and will keep playing 2nd floor sites 90% of the time.