This discussion has been held in many different forums and the answers are generally
obvious because of the dictations of the physics involved.
There was even an engineering study on the subject.
Although there are gains do to reduction in drag ,The biggest predominant factor is
the energy required to achieve orbital velocity.
Buts lets make it even simpler ,here is the economics of it.
The pegasus vehicle cost is $30,474 per kg
The STS cost is $10,416 per kg
The Ariane 5 cost is $9,167 per kg
The proton cost is $4,297 per kg
Vehicle costs are 100 to 1000 to 1 in comparison to the cost of fuel so the economy
of size is the biggest factor in cost.
Which essentially eliminates balloon launching and limits aircraft launching
to small NIRTS payloads.