Because wavelet frames also have very desirable properties. For example:
1.) wavelet frames can be much more directionally selective for image processing than orthogonal or biorthogonal wavelets.
2.) wavelet frames can be shift invariant whereas an orthogonal or biorthogonal wavelet transform is not shift invariant
3.) wavelet frames (as is true of frames in general) are less sensitive to corruption (erasure) of coefficients
