【改错】Small as it is,but it is very beautiful.为什么去掉but?

Small as it is = As it is small 尽管它很小
这是一个as引导的让步状语从句,其后的but it is very beautiful为主句.
此处的as = although,意思是“尽管,虽然,即使”.
而这个主从复合句当然只能有一个连词,用了as就不能再用but了.
类似的是：because和so不能连用,although和but也不能连用.
there be句型中的be的数采用就近原则,第一个名词是blue sky为单数,当然也就适应is而不是are了.