Like Flafpert said. The typing. Its typing is very bad defensively. It has only one resistance. Also, despite the massive 184 defense, its special defense is abmysal. Special attackers such as hydreigon, magnezone, even a mixed metagross, can absolutely ravage this thing. Plus, its speed is extremely slow, so the chance that it can actually outspeed something is nest to impossible. Plus the special attack. Like Black kyurem, this guy does not get many physical ice attacks due to most ice type attacks being special. But unlike black kyurem, Avalugg has horrible special attack.
Those are the main reasons for this guys bad tier. See http://www.smogon.com/dex/xy/pokemon/avalugg/ for more details.
Hope I helped!