Since I like the start of DS for its film-like qualities, I always assumed (much as I try not to do that) that those early actors quit because parts in films, or prime time series, or plays opened up. I'd guess that the gap between Sam Evanses (and Matthew Morganses) indicates that the loss of the actor was a surprise to DC, who might have looked for and found a new actor while MA was still there, if he could have. But then, DC was impulsive, I guess.