Why coupling reaction of diazonium salt with phenol requires pH around 9-10 but in case of aniline it is 4-5?

See how this phenol can gain or lose its proton? With water, rather than base, the reaction will tend to reverse and go to the left.   You want basic conditions for this first step, because in order for the phenol to react, and form the nitrogen bridge, you want to deprotonate the hydroxyl (and create sodium phenoxide) that will be attracted to positively charge diazonium ion and then form the azo compound.

The diazonium ion is often prepared from aniline and nitrous acid in an acidic solution. Your goal is to keep this reaction to the right and promote oxidation (without making nitrobenzene, but this often requires a metal catalyst): 

Then you cool the solution and add the cold benzenediazonium chloride.

Diazonium ion is an excellent leaving group, so if you did the entire reaction step in water, what would happen is that you would simply substitute the -N2+ with an -OH group via Sn1, rather than making your desired azo compound.