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.
I think part of their thinking is, with v2 to be released in the middle of 2019, that everyone will switch to markets based on stablecoins. They see that any market left around on ether will become ghost-towns and they are trying to prevent that from happening.
As all major current Eth markets will have stablecoin versions of them made then, making all the old eth markets obsolete, so by limiting the duration of current new eth markets they are preventing dead-end markets from laying around too long.
(There is nothing that can be done for all the existing longterm eth-markets out there already created.)