Reading Terminal - why did it close?

Can anyone tell me why SEPTA closed the Reading Terminal? I've read everything from no one was using the train, to SEPTA was broke, to SEPTA built a new station that merged all commuter lines and made Reading Terminal obsolete.

The Reading Terminal was closed because it was made obsolete by what is called the Commuter Tunnel that connected the former Reading commuter lines with the Pennsy commuter lines. Both of these lines had been taken over by SEPTA in 1976. It was now one system but the former Pennsy lines ended at Suburban Station at Broad St. The former Reading lines ended at the Reading Terminal 2 blocks away. The Commuter Tunnel connected these two lines. Suburban Station was below street level. The Reading Terminal train shed, where the tracks ended, was above street level. The lines were connected below street level so the train shed became part of the Pennsylvania Convention center and the stations were moved below ground and provided access to the Gallery shopping mall next door to the Reading Terminal.
The Reading Terminal building is still there. The Reading Terminal Market, where farmers would bring produce to sell in the city by train, is still there. The trains run underground and the SEPTA system is now connected.
I'm sure that others can provide more details about why and how the plan was developed this way.

Well, trackhiker has covered alot of the main points. Remember too that the RDG Terminal was a stub-end facility one floor above the market. That's very hard to integrate into a thru network that linked RDG and PRR tracks, especially when that trackage is underground.

Trackhiker definitely nailed it.

The good news is that the Reading Terminal Market (which actually outdates the terminal itself), trainshed and headhouse building have all been preserved and will be with us for many more years. The viaduct leading to the terminal is also still intact from Vine Street north to Green Street and will hopefully become a lateral park at some time in the future. The bad news is that the tunnel has virtually eliminated any chance of future diesel operations without the institution of significant ventilation system changes and/or some sort of dual-mode equipment, neither of which are likely in this lifetime.

The bad news is that the tunnel has virtually eliminated any chance of future diesel operations without the institution of significant ventilation system changes and/or some sort of dual-mode equipment, neither of which are likely in this lifetime.

Fuel cell-hybrid railcars maybe? I'm investigating the idea in my spare time; I think it might hold some promise.

The Reading train shed was (is?) the biggest building of it's type built at the time.
Question: What "type" of building is this, and is it still the biggest of that type?

I used to take the Reading Line to college before the commuter tunnel. I then had to get on the Market - Frankford EL to get out to 30th St. ( I'm dating myself) After the first year, I drove to Chestnut Hill and took the Pennsy Line to 30th St. Now you can just take SEPTA all the way to 30th Street.

As far as I know, the Reading Terminal is the oldest (if not the last ), and certainly the largest surviving single-span train shed in the U.S.