Why only porphyrins, but not their isomers, in nature

In 1929, Hans Fischer completed the synthesis of hemin (also spelled haemin). He was awarded the Nobel Prize for chemistry for 1930.  

It has been shown that the free space for metal coordination is smaller in porphycene than in porphin (283x263 pm and 289x289 pm, respectively), this being the reason for somewhat higher probability of metalloporphyrin formation. Thus, it was shown that the earlier claim that the porphyrin isomers cannot be prepared because they are unstable is not correct. 

Very soon, other isomers of porphyrin and the analogous macrocycles were also synthesized corrphycene, hemiporphycene and Al-confused porphyrin. 

After preparation of various porphyrin isomers, a large number of papers were devoted to the study of the influence of geometry, the size of metal ion, steric interferences, and macrocycle deformations, with the purpose to offer the answer to the question why only porphyrins in nature. 

---