It's all about momentum. By the time they would make a series with the Enterprise, Disco Fans will have already had a full season of Pike. It would make more sense to go with Pike at that point. Most people would be like, "Who the Hell is April?" Whereas, to stick to examples from this century, Pike would already be known from the Abrams Films and Discovery.
Usually a spin-off will star someone seen in the previous series. "If you want to see more of Pike, check out this new series! If you like Discovery, you'll love ______!"
Usually a spin-off will star someone seen in the previous series. "If you want to see more of Pike, check out this new series! If you like Discovery, you'll love ______!"