There are a lot of desirable things about Paladin in general. Ret Paladins do not lack utility in M+. Their utility in raid is nonexistent due to its counterparts, Holy and Protection - which bring everything a Ret brings and more.
There are essentially three things an M+ choice can bring: damage, synergy, and buff coverage.
Great tuning is the first and most important aspect of M+. Damage makes the timer faster. Damage covers up mistakes - the longer mobs are alive, the more likely you are to die. Secondly and nearly as important, synergy. The spec needs to synergize with other meta choices.
Ret needs to do things that other meta specs cannot do. In regards to stops, Rets really only have a kick and Blinding Light. It’s difficult on a lot of weeks to take quick HoJ - and even that is less desirable in a lot of ways compared to an AOE stop. For example, Incorporeal makes it quite hard to take the quick HoJ. The positives are blinding light, hand of sacrifice, BoP, and Freedom. The last two are good, but not game breaking. Most of the time these utilities are bonuses for a group when Protection or Holy Paladin are meta choices.
Buff coverage is something that plagues a lot of classes. Andybrew, at Blizzcon, was talking to Ian about how BDK will never be meta unless it is tuned through the ceiling (see S3/4 SL). Why? It really doesn’t bring anything for a meta comp. Grips are nice, but not game breaking and AMZ is good but not a pulling factor for the spec being meta. In the same way, Ret aura really doesn’t push Ret over the edge. 2% damage and healing (now that it is fixed) is pretty good, but there are other buffs that are generally more desirable and fit into the “synergy” category like DH buff and MotW. Mage buff is also quite strong, as a side note.
To sum it up, Ret brings bad damage, a competitive buff, poor mob control, and zero to great utility depending on the tuning of Holy and Prot - it brings nothing unique to those specs. Divine shield is also a plus - along with the specs tankiness.