Why is Polonium the only metal to crystallize forming Simple Cubic Cell?
The inorganic crystal structure database (ICSD), by specifying that
      the structure contains only a single element, you can reduce the
      number of hits to 55. Structure examples are Cr, Po, Mn, Sb, Se, P,
      Ca, Li, As. Some are specific phases occurring at different
      temperature or pressures.



So Po is not the ONLY metal to have a simple cubic structure, but
the other cases require special conditions.



  The origin of the stabilized simple-cubic (SC) structure in Po is
  explored by using the first-principles band calculations. We have
  found that the prime origin is the inherent strong spin-orbit (SO)
  interaction in Po, which suppresses the Peierls-type structural
  instability, as usually occurs in p-bonded systems. Based on the
  systematic analysis of electronic structures, charge densities, Fermi
  surfaces, and susceptibilities of Se, Te, and Po, we have proven that
  the stable crystal structure in VIA elements is determined by the
  competition between the SO splitting and the crystal-field splitting
  induced by the low-symmetry structural transition. Our study suggests
  that the large SO interaction would suppress the Peierls instability
  which is generally expected to occur in one-dimensional conductors.


  In fact, Po becomes airborne with ease: if a sample is heated in air
  to 55 °C (131 °F), 50% of it is vaporized in 45 hours to form diatomic
  Po2 molecules. (The melting point of polonium is 254 °C and its
  boiling point is 962 °C.)



Putting it all together, Po is in the column with Se and Te, and just to the right of Bi, none of which are typical of "normal" metals. The formation of $\ce{Po2}$ molecules at near ambient temperatures suggests that the outer electrons of Po form bonds of a highly covalent nature. 

But it is indeed unusual and fascinating, and if Po were not 5000 times more radioactive than radium, I might be tempted to do a little chemistry. 

References


Quora:
"What are examples of simple cubic crystal structure?" Gervais Chapuis, Nov 3, 2016
B. I. Min, J. H. Shim, Min Sik Park, Kyoo Kim, S. K. Kwon, and S. J. Youn, Origin of the stabilized simple-cubic structure in polonium: Spin-orbit interaction versus Peierls instability, Phys. Rev. B,  73, 132102 – Published 7 April 2006
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.73.132102
https://en.wikipedia.org/wiki/Polonium