Lucentis can be cost-effective for maculopathy (retinopathy in the central area of the retina called the macula), but only if the maculopathy is due to a certain level of macular oedema. This may be measured during an OCT scan. If the retina at the macula is 400 microns or thicker, Lucentis may be cost-effective.
Visual impairment in diabetes may be due to other reasons than macular oedema such as retinal ischaemia (reduced blood flow) - this would not be improved by Lucentis.
As others have said, you need to ask your consultant the particular reason why you have not been offered Lucentis.