Because he's not a credible leader of a government, or even the opposition. He's more suited to organising protest marches than actual grown up politics. He commands no respect from his MPs and doesn't even care. I think the country desperately needs a credible opposition to ensure that things are debated thoroughly and from all perspectives, especially at times of huge change, and we just don't have that with him in charge.
Also, as luna says, until the Labour Party sorts itself out, in Scotland we're stuck under the boot of the SNP and their Neverendum hell.