irule not being executed

The biggest issue I see is the conditional matching. The [HTTP::host] command returns the Host header value in the request URL (ex. www.domain.com), while the [HTTP::uri] command returns the URI portion of the URL (ex. /xxx). Your evaluation is trying to match the host name to both the host and the URI, which would never work. An example workaround might look like this:

when HTTP_REQUEST {
    if { ( [string tolower [HTTP::host]] equals "xxx.xxx.xxx" ) and ( [string tolower [HTTP::uri]] starts_with "/xxx" ) } {
        pool POOL-XXX
    } elseif { ( [string tolower [HTTP::host]] equals "yyy.yyy.yyy" ) and ( [string tolower [HTTP::uri]] starts_with "/yyy" ) } {
        pool POOL-YYY
    }
}

The above should work, but it also does present a problem. What if the host is xxx.xxx.xxx and the URI is not /xxx? You should attempt to account for all of the possible ways that a client can make a request. Example:

when HTTP_REQUEST {
    switch [string tolower [HTTP::host]] {
        "xxx.xxx.xxx" {
            if { [string tolower [HTTP::uri]] starts_with "/xxx" } {
                pool POOL-XXX
            } else {
                 ...what to do if the URI isn't /xxx?
            }
        }
        "yyy.yyy.yyy" {
            if { [string tolower [HTTP::uri]] starts_with "/yyy" } {
                pool POOL-YYY
            } else {
                 ...what to do if the URI isn't /yyy?
            }
        }
        default {
             ...what to do if the Host is neither of the above?
             drop?
        }
    }
}