Question

Capacity to regenerate decreases with increase in the complexity of organism. Why?

Answer 1

Evolution made life more and more complex. The more complex an organism became, the more different tissues and cell types it has, the more difficult it is to regrow them. For example a sponge has about 3 different cell types. If you take a small piece of a sponge, these 3 cell types well still be present and once they grow they can form a whole new sponge. The more complex you go, the more different cell types you need, and thus the harder it becomes for a few cells to represent a full organism.

It also has to do with symmetry of the organism. Sponges have none, cnidarians have radial symmetry, mammals bilateral.

As for the lizards, they can "regenerate" a tail but it is never the same tail as before, it will always be shorter than the one they lost. Also the tail is a simple part of the lizard, and hence can be regenerated.