Arguably a better location to interface SEPTA urban transit with the NEC would be where the Market-Frankford line crosses over the NEC.
When you said this, I immediately thought of 30th St. Station. I have no idea why the direct, covered, indoor connection between stations remains closed.
Then I realized you meant the *other* place it crosses over the NEC, at the northeast end.
Also a good idea...
But probably reopening the direct connection at Market St. would be relatively easy to do, and I really have no idea why it hasn't been done...
It may not be that easy to reopen the 30th Street - MFSE connector. The direct connection was originally closed due to the number of twists and turns that made security down there an issue that neither Amtrak nor SEPTA wanted to address. Yes, that was a copout, but that was the reason.
Later, as part of the renovation of the station that added the food court and restaurants, the stairway into the station was completely removed and the marble flooring was extended over the area. Other than a very slight variation in the color of the marble, the location of the old SEPTA MFSE stairway within 30th Street Station is hard to even find today.