In the current market it's going to be phenomenally difficult for a new site to get going without being a 'skin', sharing users with an existing network, because people will want enough opponents to play against or generous enough offers to make up for the lack of table selection options.
The skins act like glorified affiliates - so the actual network is the same, and there's just some cosmetic differences, and they take a cut from the rake.
There are various advantages for skins existing for players, though. Generally you can sign up to multiple skins for the same network, and take advantage of several sign-up and 1st deposit bonuses - if you find a network you like you can then keep playing at the same tables but get extra bonuses. Additionally, you'll get more players at the tables than you would if they were separate, so it's more worthwhile playing there.
The network running all the skins gets users from several marketing efforts, while the poker sites
get a cheap way to enter the market (they can get started from maybe $10,000 plus some marketing costs, which is a tiny fraction of the cost of starting up a new independent site).