Safety, scope of mission, budget

The first moon missions were extremely risky. Many people died during development and/or during those missions. As NASA evolved, they reprioritized safety and that changed the demands of the vehicle.

The scope of the missions are also different. The first moon missions were, mostly, about just getting there. Taking moon samples back and doing science experiments were limited. The mission now is for a larger group of people to stay on the moon for week/weeks.

Budget has been a huge issue since the 80s. Once the allure of new and exciting space things died down after the first landing on the moon, public perception and federal budgets got moved to other things. The reason NASA is using the SpaceX rocket isn’t because they couldn’t make something better. But SpaceX has done a lot of the development on their own dime. Getting a moon-worthy rocket without an additional decade of funding and research ensures reasonable timeframes for the new moon missions.