The reason that exercises which begin with an eccentric phase are generally easier to progress on and can usually allow for more weight to be used is due to the eccentric loading of the muscles and connective tissues. I believe this is known as the "stretch shortening cycle".
Eliminating this build up of potential energy that gets stored in the connective tissues (mostly tendons) by beginning the exercise from a stretched position forces the muscles to do 100% of the work.
This is the reason why when you see someone do a set of really heavy deads, the first rep looks painfully difficult, but once they get the weight up the following reps appear much easier (well except for the last one if they are going to failure).
Of course there are benefits to both methods, so I would personally suggest using which ever method you felt was more conducive to your goals.