[I'm Founder of isocket, the worlds largest marketplace for "premium" ad inventory]

This question assumes (perhaps wrongly) that 1) a SSP and an exchange platform are all that different and 2) publishers don't understand what SSPs do and are staying away from them due to ignorance.

It may be that some do understand and chose not to go that route. More and more premium publishers are improving their strategy by reducing the amount of inventory they make available in any of the remnant spot markets.

An SSP is basically an exchange of exchanges. Their job is to make the most out of your remnant inventory. An exchange has the same goal. And many of the exchanges expose your inventory through the same RTB pipes that an SSP would.

But just speaking about a situation where someone is defaulting to a single exchange out of ignorance, it could be any of the normal reasons that create early adopters vs late adopters, such as comfort in the known, perceived switchover costs, tradition, internal corporate bureaucracy, turf battles, people just not giving a crap, etc etc etc.

Also: Publisher's fear of channel conflict, price erosion, and data leakage are all completely valid - but sometimes publishers take this too far and make seemingly bad "ignorant" decisions based on fear and inaccurate info.