Access to the regulated market costs a lot of continuous administrational effort (very specialized human resources) and money. Not all players in the general market go the full way and therefore have very restricted or no access to the regulated market. Also, information is not yet diffusing uniformly to all players as many of them would not even know where to start looking for it.
The mix of those two fundamental reasons provides for lots of market arbitrage opportunities for those with cross commodity access and the sophistication to exploit them then in financial trading. The physical link, which very often does not exist in the financial market, provides for further opportunities that can be exploited.