That's the second time you allude to these 'incentives' without going into more detail. If you could elaborate I'm sure it could help the thread going forward.
Well, I did mention the BGS and reputation. If you want to flip a system with economic means or gain reputation and all they buy or sell is scrap, it's scrap you're hauling. Of course there's substitute activities to play the BGS, but economically, scrap is as good as palladium.
Second is the composition of trade routes and your own profit goals. Mine are to pack as much profit in as little time as possible, which means I can afford to fly grain on a return trip and will not deplete small market-high profit sources on my own, since I'm usually done for the day when prices drop and profitability suffers (I just make sure to update the trade network with the prices I get after I've finished my business ;P )
Low profit good's contribution profit margin is as high as that of palladium. But only because the running cost of trade vessels is so low.

(and it's only meaningfull, if you care about junk like contribution profit margin and somesuch).
And the existence of those trade routes is no secret. There's even a post here with "ZOMG 4k per ton profit in one jump" (<- that's not the average profit). They just don't usually show up in the trade tools, since they're small markets that get trashed fast. The stable stuff for ok-ish but sustained profit does show up.