为什么数字通信一般都采用正交调制

This is indeed a fundamental question in digital communications. Leaving In-phase and Quadrature aside, the main purpose of using orthogonal "carriers" (either in terms of different time slots, different frequency carriers, or different code in the CDMA context) is that by doing this, the component carried by each "carrier" can be perfectly recovered.

Now coming back to the I/Q business, if we restrict the tranmitted signal to be bandlimited over to frequency band [-W, +W], then there the degree of freedoms we have in manipulating this family of signals is 2W per second, ie, each such signal can be represented by 2W numbers per second. This very fact also governs the Nyquist rate of sampling, although most people tend to understand Nyquist rate only from the Fourier Transform perspective. (which is indeed a limited view,  Fourier transform is merely one way of providing an orthonormal basis).

When this is understood, the idea of using I/Q components is for the purpose of fully exploring the degree of freedom. Signal s=I*cos(wt)-Qsin(wt)] is such that the uppers-side band and lower-side band have no symmetry or redundancy, so that both upper and lower side band can be used to carry information.