I'm guessing because years ago, you didn't have all the incidental crap outside of the actual design and construction of the bridge. Yes, I realize there had to be some but now-a-days, things get so bogged down in the minutia that people lose sight of the end objective. Just to get a job to bid, there is an extensive amount of money and time spent in environmental items (far more I believe than in years ago). Gov't's now hire mostly outside consultants to consider design options and alternatives. These consultants are often a drain on funding because there is little cost oversight. And when they run out of money, the gov't just funds more. I see this daily. Once you get into a legitimate design, there are all the new environmental restrictions put in place which drive up cost tremendously. Erosion and sediment control on these jobs is outrageous and way over the top. And it's not just the actual application of these elements, it's the paperwork required of it. It's also the sequencing of it. Hell, I've seem jobs take twice as long simply due to the phasing dictated by E&S management. That's a huge cost that has to be covered by contractors and it's an asinine way to manage work.
You also have requirements now that weren't in place. A large one is minimum MBE (minority business enterprise) participation requirements. This could mean anything from race to gender qualifications (even veteran status, etc.). It's the biggest scam on the books. Gov't says that 30-35% of the contract has to go to an MBE firm. So as a contractor, you're bound to this. This strips away competition and ultimately forces a contractor to use someone it may not normally often to the detriment of the work. To offset this, the contractor must add resources and funding to account for added oversight, added rework and oftentimes blank budget to simply complete certain tasks for them. We're talking huge wasted dollars here. And if these MBE companies were competitive on their own (able to work with competitive prices, provide a quality product), wouldn't they compete without the MBE status? Some do but not most.
There are a million reasons why things are different now. You'd be surprised HOW and WHERE the money is spent in a project a this magnitude.