I wouldn't have thought that if this was authorized in 1982, that 30 years later we would still not see it even being yet under construction. Especially since it is a gap in I-95. I guess they are waiting for HTFO.
I agree that the lead time to construction has been outrageously long. Among the contributing factors have been legislative delay, the usual design time, stakeholder involvement, and funding. Let me explain each (and apologize ahead-of-time for the long post).
Legislative delay: After the 1982 Somerset dump, the PA legislature didn't authorize the PTC and PennDOT to work on a new interchange design until 1985 (with PA Act 61).
Design time/funding: It took from 1985 to around 1992 for the design team to secure design funding, design a complete set of screened alternative configuration, and recruit a Community Advisory Committee (CAC). (I was on the CAC from 1992 to 2005.)
Stakeholder involvement: When the 1982 federal legislation rerouted I-95, it necessitated a high-speed interchange between the PA Turnpike and I-95 (because I-95 would be need a continuous routing through the new interchange). This forced the PTC and PennDOT to scrap all plans (the aforementioned double trumpet configuration, which had seen some initial construction) and start over again. Drawing board, square one, scratch, clean sheet (and other clichés as appropriate).
Since the new configuration was now being designed in the era of increased environmental regulations, all that had to start from scratch as well. Because building a direct connection between the two highways would radically change driving patterns throughout the surrounding area, there turned out to be a huge number of stakeholders to consider and involve.
Since the local arterial system currently supports and filters the traffic which moves from one highway to the other, the direct connection would redistribute noise and emissions. Not to mention residential and business takes in whatever location was built upon. You had environmental considerations, housing considerations, neighborhood considerations, traffic pattern considerations, this, that, and the other considerations. You name it. All represented by multiple stakeholders, all of which needed to be satisfied in one way or another.
Having been on the CAC and having had an inside view of the process for this particular project, I think the design team actually did an admirable job of working with the locals to hammer out a configuration which satisfied the most number of stakeholders in the greatest possible way. They devised a configuation which nearly everyone agrees will be OK to live with (from a local liveability standpoint) for a long, long time.
But getting all those stakeholders in line through iteration after iteration of design (responding to this input and that input) is what took so long. It was like herding cats. Or what in the military we used to call a "goat rope."
Construction funding: It has been quite thechllenge securing funding for construction. That job still isn't done.
If I had to identify the single biggest factor in the long lead time, I would say the stakeholder involvement, with funding a close second. Other perspectives may be just as valid, though.