The main issue was cost. A lot of basic research had to be done, developed, and put into production on an accelerated timescale. Control systems and processing capabilities were much more primitive which hampered reusability. Materials and fabrication technology were less efficient so we had to build big to get useful payload mass fractions. CAD modeling and computational fluid dynamics were just entering use, so designing rocket engines was as much an art as a science. You could work around most of this by just throwing money at the problems, but Nixon felt it wasn't worth the cost.