Perhaps a higher focus by political leaders over the years, due to higher exposure and regional revelance, combined with a long term embracement of poor urban planning concepts.
It boils down to downtown having no RAP, downtown being a sea of now-vacant land with associated unrealistic cost barriers to entry for the types of businesses that spur redevelopment, and last but certainly not least, downtown is and continues to be a chess-board for a handful of 5 people who use it to make money off unnecessary projects and policies at taxpayers' expense.
Riverside escaped this fate because of RAP, which prevented it from becoming a bunch of vacant lots. Contrast that with SPAR, that's the literal exact reason that Springfield didn't come back. You're to the point now over there where, especially with commercial properties, the choice is exorbitant rent at one of the handful of places still standing, or building from scratch, which nobody can really afford. Meanwhile, Riverside is booming, paradoxically, even in the face of a prolonged recession. We've had 1 demolition in 10 years. As much as Dashing Dan has made this a Riverside vs. Downtown topic, it's really just about Riverside, because the same issues become obvious when you compare it even to other residential neighborhoods. In the entire state, potentially the country, Jacksonville is 1/2 blessed 1/2 cursed to have the two historic neighborhoods that best exemplify how to do urban redevelopment and how not to do it.
Nobody wants to acknowledge the economic realities that make downtown the sea of tumbleweeds that it is. There are not enough residents, there are not enough open businesses, it's too much of a hassle to frequent the few that are open because of the parking policies, which is the same reason all the corporate offices have fled. Nothing is connected or even makes sense because of a useless 1-way street grid, and because of asinine zoning and signage restrictions it took two years of waiting for an answer about whether they could have outdoor seating before the last restaurant (that wasn't at the landing) that tried to open down there just gave up. Not that you would have known they were there if they had opened, since you're not allowed to put up most any kind of visible sign, sandwich boards, etc. Unless you're a bank or insurance company, then COJ has no problem with you putting up 40' high lighted signs on all 4 sides of your building.
All of that needs to be addressed immediately, along with moving the vacant lots into redevelopment mode. None of this is fixable by building another boondoggle like a convention center. In fact, that will simply fail too. The environment down there is toxic to business and visitors, and the toxic environment needs to be cured first before any of this will happen the way any of us want it to.