Board Thread:Questions and Answers/@comment-116.65.70.72-20130727121600/@comment-5527544-20140112034155

The longer an Ultra is 'dead' the harder it is to revive them, also it was stated that an Ultra coming back to life, no matter how many times they do it 'miraculous' since it is not a given it will work.

Also you ask question to which the answers are obvious, it was Ultrama who was dying and it was his life force that kept Hayata alive. Zoffy gave both Ultraman and Hayata a life force so the could live seperately but Ultraman still needed treatment, so keeping them together would've endangered Shin's life.