why do Mammals try to maintain a blood pH of 7.4?

Answer 1

its the optimum pH that their enzymes can work in
change in pH may cause some proteins to denature and break down

Answer 2

I imagine that its the optimal operating pH for mammalian haemoglobin, though I could be wrong about that. All enzymes have optimal operating conditions, often quite different from other different enzymes, and deviation from those results in less efficient functionality. Deviate far enough and the proteins begin to denature... though that would have to be quite a change...

Answer 3

That is the pH that works best.