I think it may be just a change of priorities. At the time of TOS tensions were high with both the Klingons and Romulans, and much of their first contacts didn't necessarily start too well (e.g. the Gorns on Cestus III).
By the time of TNG a lot of things have stabalised. The Klingons are allies, the Romulans have vanished again, and certainly although there are still conflicts, they seem to be small bush-fire affairs. Starfleet also appears to be much more vast than they were even in the movie era, so it could simply be a manpower shortage. It's easier to get civilian scientific experts to sign up for exploratory missions with Starfleet that to get them to actually join and go through the Academy.
By the time of TNG a lot of things have stabalised. The Klingons are allies, the Romulans have vanished again, and certainly although there are still conflicts, they seem to be small bush-fire affairs. Starfleet also appears to be much more vast than they were even in the movie era, so it could simply be a manpower shortage. It's easier to get civilian scientific experts to sign up for exploratory missions with Starfleet that to get them to actually join and go through the Academy.
