becasue 表示“肯定的原因”,没有猜测在里面.
而这句话中用了个“could”表明有猜测之意,是可能的意思,而不是能够的意思,所以用as,as表原因时有猜测之意,跟for表原因类似.例如：
there is water everywhere,because it has rained.因为下过雨,所以到处都是水
There is water everywhere,for ( as ) it has rained.到处都是水,大概是下过雨了吧