Crown corp would be great but letting them monopolize the entire process might not be the optimal thing to do. In some cases it worked well (e.g. hydro-quebec prices are very competitive) but in others it gives them unlimited power to set the prices as they want in order to achieve a target profit, with potentially murky decisions like automatic bonuses and millions in severance payments.
Oth crown corps competing in the market allows it to be more efficient since other players cannot push crown corps out of the market through acquisition, yet crown corps have to adapt their practice to be competitive and lean; good example is CDPQ infra participating in a competitive market when building light rail systems across Canada/UK.