It's been done many times in the past. Anyone is free to create and run a copy. The issue they hit is that no one cares, it doesn't gain traction...

A different chain sharing a near-identical codebase (with a different genesis) is possible but Bitcoin is much more than that now. The hash power that has been put into the mainchain can't easily be replicated.

A competitor starting from scratch would have to either choose a new POW algorithm or be at the mercy of attacks from miners on the mainchain, assuming there is liquidity in the competing market. And when I say liquidity I mean a market for local currencies since miners have expenses that can't yet be paid for in Bitcoin.

An alternative to that happening is the case where a majority of miners switch to the competing chain, but that's unlikely for a number of reasons. First, why would a majority of miners leave the mainchain to pursue a competing chain with no liquidity? Following that line of logic, why would Bitcoin holders leave the mainchain for one that offers no added benefit? A majority of miners still don't equal the sum that were on the mainchain so the reliability of each transaction would be reduced.

Now if we're discussing hard forks, there's another set of reasons why they're not successful. All of them start with the fact that the changes made are not desired or needed (as indicated by the market of existing hard forks).