Because there's a higher risk that both strings will break in the same session if you rotate them.
Now, "higher" does not mean "high": the chance is still quite low. It's probably more of a personal preference.
Yes, I remember when I had several frames where the strings were just about to go on them. I decided to bring them all out one day. After breaking the strings on the 3rd or 4th racket, my opponent was amazed and asked how it was possible to break strings that fast. I did tell him that they were all about to break when I started playing and not some magical power I had.
I do rotate my rackets. I have 3 microgel prestige pros and 3 youtek prestige pros. I've got 2 gut/polys, 2 poly/multi, and 2 with worn strings that I just use for practice. Once the newer ones start to get worn, I go ahead and practice with the ones with worn strings until they break or the poly is very dead and string them up new.
I also have 1 PS 85, 1 Yonex RD Ti 70 88, 1 Yonex RD Ti 70 98, 1 POG, and 2 Kennex Black Aces I accumulated over the years that I practice with a lot. All of those are strung with multis, so I have to get those restrung when they break.
Just got 2 ProTour 2.0s for better control with the gut/poly than the PPs with gut/poly, so those will now be my primary rackets with the PPs as backups.
Really liking the PT 2.0s with the gut/poly hybrid. With the PPs I played best with the poly/multi or all poly, but the gut is easier on my arm.
Always have a backup non-gut racket in case you have to play in rainy conditions.