Well, one reason is that there are models that make use of something like total utilitarianism to great effect. Classical economic theory, for instance, models everyone as perfectly rational expected utility maximisers.

I think you actually slightly understate the case against Utilitarianism. Yes, Classical Economics uses expected utility maximisers - but it prefers to deal with Pareto Improvements (or Kaldor-Hicks improvements) than try to do inter-personal utility comparisons.