Because the tolerances for time keeping and getting the 'phases' to match up are about 1 million times more severe at the shorter optical wavelengths than at radio wavelengths. Also, because the wavelengths are also shorter by about 1 million times, maintaining a fraction of a wavelength tolerance in the spacing of the optical wave front receivers becomes a daunting technological challenge. Astronomers have only now begun to build instruments and 'optical arrays' a few dozen meters across. Compare this to the 27 kilometer 'Very Large Array' radio telescope built in the 1970's.