Delayed second review of a reputed journal I had submitted an article and I got first decision. I wrote answers and explanations to the reviewer comments and re submitted the article. After 2 months it sent to the same reviewer (as per info given at help section) and i am told that the reviewer has not accepted the invite. It is very surprising, why would the same reviewer who reviewed my article once would not accept the invite to see my answers. Or why would the editor wait for almost 2 months for the reviewer to just accept the invite the review.
After 2 months it sent to the same reviewer (as per info given at help section)....[W]hy would the editor wait for almost 2 months for the reviewer to just accept the invite the review[?]


The timeline seems to vary a lot by field, but sometimes the journal publication process is extraordinarily slow. On the whole, 2 months doesn't seem out of the ordinary to me. If it was just the editor waiting to ask the reviewer, I agree that seems strange, but there may have been some good reason for the delay (e.g. back and forth with this reviewer to see if they would be able to accept it or not).


  and I am told that the reviewer has not accepted the invite.
  It is very surprising, why would the same reviewer who reviewed my
  article once would not accept the invite to see my answers.


Unexpected things can happen. For example, the reviewer had some personal tragedy or health emergency. You simply cannot tell, so the best thing you can do is to discuss with the editor of the paper what to do next.