Bot has two people so a jungler coming to gank doesn't necessarily change things as much when it's a 3v2 situation. That and one of the bot laners is typically dedicated support and AD Carries that are put bot are ranged and play at safer distance so the lane itself is just more protected overall.
Conversely, top is a solo lane so 2v1 situations are just harder to deal with so junglers can make a huge difference here. You also tend to have melee champs here so getting caught out of position is a lot easier as you go in farther to effectively farm.
So it's basically metagame stuff, I suppose. Top is more exposed and hurt more by jungler presence so it's the easier lane to gank. This also isn't that relevant at low summoner levels and probably even low elo play, however.
Thanks I was also wondering about why exactly it's common to have 1 on top and 2 on bot though? Isn't it pretty much symmetrical?