There was a time when getting a bond of 20 MPa to enamel and dentin was considered a success. Now I read that some bonds to enamel and dentin are in the 30s and more. Why are the bonds that high?

Newer bonding agents with higher bond strengths to tooth structure and other substrates contain variations of MDP. This phosphate-based monomer has a high affinity for tooth structure, metal and ceramic oxides and this results in a higher bond.