The H-2 (proton+neutron) bound state is stable. Nuclear forces are nucleon-independent, so two neutrons should interact in the same way as a neutron and a proton, since in neither case there is electrostatic repulsion. However, the dineutron bound state is unstable.
Is it related to the Pauli exclusion principle? Possibly analogous to Hund's rules? A neutron and a proton "fill up" different shells since they are different particles, but two neutrons would fill the same $1s$ state. If we followed "nuclear Hund's rules", the second configuration would have a lower multiplicity since it's a singlet state and therefore it would be of higher energy.
