When using SPDY/HTTP2 profile, TCL variables set before the HTTP_REQUEST event are not carried over to further events. This is by design as the current iRule/TCL implementation is not capable of handling multiple concurrent events, which may be they case with SPDY/HTTP2 multiplexing. This is easily reproducible with this simple iRule:

when CLIENT_ACCEPTED {
  set default_pool [LB::server pool]
}
when HTTP_REQUEST {
  log local0. "HTTP_REQUEST EVENT Client [IP::client_addr]:[TCP::client_port] - $default_pool -" 
#  - can't read "default_pool": no such variable while executing "log local0. "HTTP_REQUEST EVENT -- $default_pool --"
}

Storing values in a table works perfectly well and can be used to work around the issue as shown in the iRule below.

when CLIENT_ACCEPTED {
  table set [IP::client_addr]:[TCP::client_port] [LB::server pool]
}
when HTTP_REQUEST {
  set default_pool [table lookup [IP::client_addr]:[TCP::client_port]]
  log local0. "POOL: |$default_pool|"
}

This information is also available in the wiki on the HTTP2 namespace command list page.