In essence ASX CFD's are the same as OTC CFD's, the ASX product is offered by the ASX but not on an exchange - it is an OTC product.
They are both a contract between two parties and cannot be bought from or sold to a third party.
The ASX is competing against other CFD providers in the marketplace.
I don't trade ASX CFD's simply because there are providers out there with cheaper commissions and margins.
Liquidity is not a concern as it is a contract between you and the product provider only, there is no "issue" of units or shares like in an IPO. The product provider can open as many individual contracts as their risk management allows.