When soliciting bids for an energy supplier, having more expected participating residents and businesses in the program will allow bidders to offer a larger discount on their bid rates.  Using an “Opt-In” program would result in a much smaller number of participating residents and businesses than an “Opt-Out” program and would likely result in a higher bid price.
Factoid: If you look at your energy bill, you’ll find that National Grid has already “opted you in” to an energy supplier that they have negotiated with already.  You can “opt-out” of this National Grid selected supplier if you wish and contract with a supplier of your own.