There are probably as many answers to this question as there are ScrumMasters (25'000 the last time I looked), but two factors stand out:
- dealing with complexity
- maintaining agility of the team (and by extension of the company).
I like to refer to the agreement between prodoct owner and team as the "Sprint Contract". So arbitrarily changing one of the parameters violates the Sprint Contract, and this is clearly not allowed.
Harry Sneed's "devil's square" shows the relation between Scope, Quality, Time and Cost.
And the end of each Sprint, the Product Owner inspects the progress of the team -- defined exclusively through the amount of scope produced -- and has the option of changing the course of the team.
Changing any of the other parameters, say the duration of the sprint, is 1) a violation of the sprinnt contract and 2) hides from the project owner that progress is not being made as planned (or hoped). He is deprived of the chance to make a course correction.
I believe there is a close correlation between corporate agility and fixed length sprints, so the next question should be, how long to sprint...