Because if they just improved Dinoland, they'd still have to come up with an attraction to replace Primeval Whirl, which had a small footprint. They'd still need to close more of Dinorama to make way for a new attraction in this space, especially when you consider that the original plans for Dinoland would have had the Excavator roller coaster take up all of the Dinorama space (TriceraTop Spin to Primeval Whirl) By that point, you are already developing half of the Dinoland, so might as well redevelop the whole land into something new. They could have split the land in half like the Moana/Zootopia Blue Sky, keeping Dinoland in the South and Tropical Americas in the North of the land, but then they'd need to create another resetaurant and another shop for the North to make it a "land".
I'm going to miss Dinosaur as I can't do rollercoasters, but Dinosaur had enough thrill in it to make it more exciting than just a classic dark ride. At least Indiana Jones should be a similar ride vehicle experience, just with different scenes.