People are just plain weird to harbour grudges to that extent but then we're not a very nice species when it boils down to it.
In fairness to Elizabeth I I think it ought to be pointed out that Mary's imprisonment was in the main for purely political reasons and there had been many times when her execution was imminent during those 20 odd years of incarceration which Elizabeth quashed quite stoutly as she knew that Mary could not be and was not involved in any conspiracy. There was certainly no love lost between the pair, but Elizabeth did not really want Mary executed.
What Mary was however was a rallying point for the Pope and prominent Catholics who wished to end Elizabeth's reign. After one too many assassination atempts on her own life, Elizabeth finally signed the death warrant and thus removed any more threats. With the succession shifting incontravertibly to Mary's son James VI of Scotland (who had already succeeded to his mother's tenure on the throne there after much policitical manoevring by the English to get a Protestant heir in place) was also Elizabeth's closest relative.
Both women were in a way victims of the times and really Elizabeth's reign was a remarkably stable and prosperous one considering the political difficulties her government constantly faced - and why she remained a 'virgin' queen so the tentative balance of international alignments could be maintained.
"Some men see things as they are and ask why. Others dream things that never were and ask why not.” George Bernard Shaw