why did capacity of regeneration decreased in evolution?
Evolution made life more and more complex. The more complex the "bodyplan" of 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. Sponges have none, cnidarians have radial symmetry, mammals bilateral. To put it simple: if you slice a piece out of a jellyfish like you would out of a pie, it's easy to imagine it can 'close' itself with new tissue coming out of the border tissue, because it's the same tissue as the one you removed. If however you cut of a foot of a human, the cells of the leg won't produce new foot cells, because they are (a lot of different) 'leg cells'. 

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 still pretty 'simple' (if you can put it that way). If you would cut of a lizards leg it would not regrow... 

I hope this has somewhat answered your question!