If you mean the mechanics of DNA itself, there have been some variations. Prior to DNA there was RNA. The evolution of DNA required catalytic precursors and mechanisms to translate between RNA and DNA and represents a major advancement in the history of life.
As far as why more extensive and variable changes apparently haven't occurred I can only speculate. There are an infinite number of ways the information of life could potentially be stored. It's an interesting topic. Different chemical structures probably do exist somewhere in the universe. Besides storing information, DNA is also catalytically active and is not as simple as a purely mechanical (punch card) style of memory. DNA is well suited for use in an aqueous environment (inside a cell) and life on Earth is strongly dependent on the molecular properties of water. If, for example, a polymer chain (say made of polyethylene plastic) was studded with "impurities" to encode information for life, it would first have to be transcribed into a form that could participate in the chemical reaction. However polyethylene is so inert in water, that it might be too difficult or impossible for such a mechanism to develop. Water would not be the solvent to use for plastic "DNAs".
Life as it is now, assuming that it could have been different, seems to be stuck with this at it's core.