Well I think it comes from the ethos, of let a router be a router, let a switch be a switch.
Back in the day - if you wanted VPN's you had a concentrator, but then I suspect Cisco started to listen to it's customers. Then VPN's and Firewalls are in the IOS of routers, dynamic routing is coded into Firewalls. Firewalls/Routers performing IPS/IDS/SPAM/SSL etc
I think it's a system of consolodation, one device does all.
