Why has the market opened up to so many retailers?

Why?

Because there's profits to be made and the barriers to entry is lowered then before.

From the proposed articles on EMA website. The Open Market is rolled out in stages and only recently to residential households. The number of retailers as it seems is to provide:

1. Competition between retailer will drive better prices.

2. More innovation may lead to new products and price plans

3. Traditionally, in many other markets there will be some form of consolidation over time but for now, seems like consumer gets to benefit from such competition.