For the same reason that HubPages does not allow flashing banners, colors, strange fonts, or active scripts--they don't want people abusing the system. Many people have asked for increased HTML and coding flexibility, but ultimately HP has to restrict what we can and cannot do within the hubs because there are too many users and they can't moderate the whole site very efficiently.
If they allowed people to insert widgets, people could use that as a window to insert harmful scripts.
Have you considered using Blogger or creating your own website? You can add widgets there and then link to the page from your hub. I have written some hubs on the topic of building your own websites.