I think partly it was because they used to be too frequent, if you stacked missions you'd have a gaggle of NPCs wanting to make offers. People mostly ignored them because they usually offered a ridiculously low reward, kind of like if you had a million credit haulage mission they'd offer 10,000 for you to deliver it to their faction instead. Eventually they just switched to using the message system insteadWhy did the NPCs offering the alternative mission offers get removed. I'd actually forgotten all about them.