The reason for this is pretty simple.
In Accounting you are not allowed to revalue your assets at fair market value until the value is actually realised at the point of sale. Rogers spent $163 million on the blue Jays and $25 million on the Rogers center. That means they have a balance sheet value of $188 million on an asset Forbes values at $1.6 Billion. By building a new billion dollar stadium it allows Rogers to revalue the Blue Jays at far closer to the real value of the franchise and since it can amortize the expenses cost in depreciation over a long period of time (30 years) they get that money as an offset against revenues to lower their taxes on an appreciating asset value.
Rogers has all the incentive in the world to spend as much as it can on a new stadium if they are comfortable with the value of MLB teams continuing to rise, it would increase the share price to add a $1 billion asset in the books.
Should also add, Rogers only needs the government for land. The last thing they will want to do is give the government a reason to insist on design input where they will try to squeeze every penny out of trying to make it multi purpose when the Jays will want to keep it in their control so they don't need to compromise on their needs.