Why do some internal organs regenerate?
It is not really essential that all vital organs should have regenerative potential (for example brain). 

The cellular composition of a tissue is a big factor in deciding if the organ/tissue will regenerate or not. Some cells divide while others do not; highly specialized and polarized cells like neurons do not divide perhaps because the act of division itself will compromise their function. That doesn't mean these tissues cannot regenerate; they would need stem cells that can differentiate into the type of cells that died.

So regeneration can happen via:


Division of the tissue cells (skin healing, liver regeneration)
Differentiation of stem cells (erythropoesis, muscle regeneration)


Organs that do not have non-dividing cells can regenerate by simply proliferating whereas others would need to maintain a stem cell niche- which is not possible in all tissues. Maintenance of stem cells also adds up to the cost of metabolism; so there is some tradeoff.