Because liberal dissidents weren't just criticizing teachings that were on the level of prudential judgments. If liberal dissidents were disagreeing on teachings that were matters of prudential judgement, I don't think the conservatives would have had much of a problem.  

Liberals however dissent on things where there is no freedom to dissent. That is the problem.  There is no freedom to dissent from church teaching on abortion, contraception, gay marriage, divorce and remarriage, sex outside of marriage, women's ordination, etc.

When it comes to the death penalty and the application of Catholic social teaching, there can be a variety of views.  Catholics are even free to hold opposing viewpoints from the bishops--and they are still in good standing.  Why?  Because these things do not involve intrinsic moral evils-therefore they do not have the same moral weight.  In other words--there is an eternity of difference between support for the death penalty and support for abortion.