Why did the large number of plane hijackings and bombings in the 70's-90's not result in the sweeping safety measures that resulted from 9/11?

They did, and the security measures were often much harsher than they are now.

The Anti-hijacking Act of 1974 was one of the first large responses to the rash of hijackings and bombings in the 60s. It empowered the FAA to come up with uniform standards for airport security.

They came up with magnetometers and scanning of luggage. Between the point that all airports were compliant with FAA rules, all individuals had to be patted-down by hand, and all luggage, both carry-on and checked, had to be manually searched.

Later, after the Lockerbie bombing additional standards were put into place which saw the requirement that all electronic devices had to be turned on and operated before they would be let in to an airplane and checked baggage screening was increased. They also implemented passenger-matching for checked bags so you couldn't check a bag and then not get on a flight.

The government had been seriously considering a complete ban on all electronic devices after Lockerbie and it is fortunate that they settled on "turn it on" for a couple of years, even if it did turn security checkpoints into nightmares.

The Air Marshals program dates from the 70s, research into backscatter X-rays dates from the late 80s, and physical inspections of all bags has come and gone as incidents occurred and faded from public consciousness.

Behavioral analysis and attempts to profile or predict attackers also have come and gone, with pseudo-scientific behavioral analyses being used in the 70s and 80s, then fading, and "Screening of Passengers by Observation Techniques" being used in the 2000s.

Airplane! and Airplane II: The Sequel, movies from 1980 and 1982, satirized the heavy-handed approach and perceived ineffectiveness of airport security at the time.

In the past 15 years the most notable changes "pre vs. post" have been the ban of liquids in the cabin over a certain size, to prevent binary explosives from coming on board, the removal of shoes so they may be X-rayed, and no-fly lists.

The liquid and shoe policies were enacted after unsuccessful attacks long after 9/11, and the no-fly list actually existed prior to 9/11 but was vastly expanded afterwards.

Also knives. One can't bring a Swiss Army knife into the cabin anymore. TSA actually tried to repeal that rule but they were overruled by Congress and the flight attendant's union.

People seem to believe that non-passengers can't go through security anymore but that isn't strictly true. All you have to do is go to the agent and ask for a gate pass. It's optional but requests are almost never refused. That policy has eliminated Hare Krishnas and bible thumpers in the terminal though so it is a net benefit.

Here is a general timeline of what happened, when, and what it was in response to: http://articles.latimes.com/2011/jun/12/travel/la-tr-airline-safety-timeline-20110612