Why gap in Mack engine sizes for years?

[Jamaican Bulldog]

Please forgive my many 'why' questions. I have been a Mack aficionado for years and discovered this site recently.

I notice for many years there was a gap in engine size between the Mack  6 cylinder engines (E6 to E7 or about 11-12 liters) and the 16+ liter E9 V8. At the same time Cummins, Cat and others had the NT855, 3406 respectively or 14 liter engines and also large V8s. 

Why did Mack go a long time without an engine size in the 14 to 15 liter range between the smaller I6 engines and the V8?

Was it because the high torque rise engines of the smaller engines coupled with the integrated powertrain competed well enough to not need a bigger 6 cylinder engine?

Or was it because of development costs, and that it was cheaper and easier to offer stop gap engines as options from other vendors?

Or could be that the market was different 25 or more years ago and the demand for the higher power 14 to 15 liter sizes came about later? 

 

[41chevy]

MACK developed a 14.5 liter  885 c.i. "big" six between 1978-1982 and never put it in production. Why? Most likely the lack of need or the coming EPA regulations.