With Sentinels, because of the Damage/Defense mechanic of the T1/T2 with the inherent, it is likely that Dual Pistols may be preferred by the more defense-oriented crowd, whereas Assault Rifle may tend towards players that prefer offensive tact. In such a case, the skipping of the juicy T2 may be to prevent defensive opportunity from triggering. As for Life Drain, it is at the thematic core of the set moreso than UT or DO, as such it makes sense that people would take it.
Finally, Sentinel is a new AT, so there may be a great many players learning it by transferring half their builds from Blasters, Corruptors, or Defenders. Also, Stunning Shot is the best single target attack in Archery, and one of the highest DpAT attacks that Sentinels have access to, but its version in Blaster, for example, is kinda less than wonderful.