Complexity...


Both sBGP and soBGP seek to protect different parts of the routing system via cryptographic means
allocation of IP addresses/AS numbers
granting routing authority for a chunk of address space
and for of the BGP messages and peerings 
note that soBGP doesn't envision protecting the session/peerings as the authors see that as a different part of the overall problem.  

That is, you (might) want to authenticate every step of the allocation of an IP address 
from IANA to the Registries 
to an ISP to another ISP to a customer
to AS number allocations
to permitting an AS to originate routes to some address space
to all BGP traffic - AS paths and next hops, etc