Now I am not able to create a market expired after 6 moths in the future using Augur frontend. Why? Does this limitation of frontend app or Augur protocol?
It is UI limitation (not a protocol limitation) meant to avoid loss of funds (lost private keys) or funds being locked up for very long periods of time after (even when the eventual result is already certain). I canot immediately pinpoint the pull request adding this limitation to the UI, but it can be found somewhere here
I cannot speak as to why that particular time period limitation was chosen (over another time period limitation) to best address the above concerns.