I am using O365, and I am in sitepages library.
I am using IE11, and Office 2013 is installed on my windows 8 machine. However still the
open in explorer is disabled, any ideas?
To answer your question about why you can’t open the Site Pages library in Explorer View and why you can open a document library is because the items in the Site Pages library are an .aspx file. This means that the items in the Site Pages library need the .asp engine to run and IIS will need to be running locally, whereas a document library hosts items as an html file which can be opened using the browser.
(Sorry for confusion, as screenshots are from SP 2010, but concepts are same.)
A little test that you can run to see the difference between the two is to create a document using notepad. Save one document as an .aspx file and the other as a .html file. You will notice when you try to opent he documents the .aspx file will need to open with a tool such as SharePoint Workspace and the .html file will use Internet Explorer to open with.