If you don't execute everyone from the old regime, there are tons of rich people, executives, special interests, influential families, etc, that remain in place and are still important after the transition.
(The post-WWII Germany was full of ex-nazi hardcore sympathizers, and nazi executives and officials in all kinds of positions of power until the 80s or so). De-nazification (not the half-arsed, soon abandoned program of the same name [1]) was a big demand of younger germans in the 60s and 70s.
Now, if you do execute all important people from the old regime, on the other hand, you have now had yourself a taste of big corrupting power over others. And unless you kill every relative, friend, etc as well, you have a recipe for civil war and further bad blood between you and those that remained.