This thread probably should be moved to the Rolling Stock forum, but anyway to add - cabooses became somewhat expensive by the late 1960s, because they carried humans, and so required safety and support measures such as safety glass for windows, cushion underframes, special trucks, grab-bars, electric stoves etc.
Even with the safety measures, there was the potential for personnel injury via slack-action (i.e. the caboose getting yanked as the slack gets pulled out of a train accelerating from a standstill - the worker can fall over and injure themselves, thus workman's comp claims etc.)
Expensive, non-revenue specialized stock has always looked upon quite unfavorably by the railroads. Couple that with the fact the railroad caboose fleet was getting old and worn out by the 1970s, and replacements (or rebuilding to new spects) cost serious money that railroads were in no real position to spend (although the Interational Car Company was turning out some really great looking cabooses towards the end in the 70s & 80s...alas, at relatively steep prices).
By the 1970s technology improved enough, and union agreements and work rules changed enough, that cabooses could be done away with (welcome the FRED, and conductors in the cab). By the 1980s, various local and state laws mandating crew size and caboose usage were mostly phased out, and so cabooses got relegated to the few duties they have now in freight railroading, such as shoving platforms or used in special moves (e.g. the nuclear flask train with a trailing caboose supposedly full of armed guards).
To sum up:
Cabooses, c 1980:
Specialized, Non-Revenue
Not really required for MOW service
Replacement cabooses are expensive
Changes in business patterns,technological advances, and laws mean cabooses are no-longer routinely required
Even with saftey equipment, personnel can still suffer injuries in them.
And that's why, once the ball got started rolling, cabooses disappeared from most railroads fairly quickly.