Because it would be incredibly difficult to make the PT look like the OT without also bringing the OT more up to date. Would you have preferred him to delete most of the ships in the PT battles rather than adding them in the Yavin battle, which he did seamlessly? Or how about the holographic displays? Would you rather he stuck with the displays in the OT which are far below the level of what we have today, in what is supposed to be a more advanced society, and instead removed the PT displays and made them look like they were from the seventies (or earlier, since the PT takes place between 30 and 20 years before the OT, in-universe)?