Anthony of Squarespace says:
Squarespace’s interface is just too advanced now to support those users. This also speaks to the sort of person we’d like to cater to. We’re not going to degrade experiences for the top 90% because of the bottom 10%—that’s ridiculous, especially considering that 10% also represents the highest tech support cost and lowest technical competence. Throwing out IE6 is a great thing. –Anthony Casalena