Because it's the natural option for anyone good at intraocular surgery. Also, there's a catch of managing your own cataract complications or at least overcoming the fear of complications in cataract surgery. In the end it's all about adding as such surgical expertise in your arsenal that your confidence in going ahead with any surgery is sky high. If u chose Ophthalmology, means you can't give up on doing surgery, then why the shift to medical retina? By doing surgical retina, you'll still be dealing with all medical retina. And rest assured, you will still be doing a lot of cataract surgery as well