They are called pescetarians and it’s a compromise. It’s a way to easily get a high quality protein source, and a way to be able to easily participate in the social gathering known as “dinner” (although most restaurants now have at least one acceptable vegetarian dish). People also tend to empathize more strongly with mammals. Most fish are free caught rather than the product of mass production that includes a cannibalistic diet, growth hormones, and antibiotics. The obvious, glaring issue, of course, is the overfishing of the oceans that has resulted in a crash in the populations of many fish stocks.