You may also want to consider another possibility, as an answer to your last question: a writer expresses himself (or herself) in the modes and forms that his age provides him with.
Some geniuses can stretch those forms, or maybe adapt forms and techniques from other cultures, like the English did with the sonnet.
My impression is that Vittoria Colonna, not being innovative, expressed herself in a way that she mastered and that was socially approved.
In a sense, her readers asked for more.
Additionally, the Colonna family was a very powerful, rich and visible part of Roman life. Hardly the context for breaking forms and declaring that she hated her late husband's guts.
Talking of which, it is very good to be cautious about interpreting feelings and moods across five centuries of societal change. In Vittoria Colonna's times, marriages among noble people were always arranged marriages. The issue of love was quite lateral to the marriage business, that was about forming alliances and producing offspring.
Finally, poetry is also an intensely personal and entertaining game. Vittoria Colonna, within her domain, was good at it; and she probably enjoyed all the admiration.