The missions back in those "good old days" of space travel were much simpler both in scope, technical complexity, and duration. The longest of the Mariner missions, for example, was about 8 months, and it was only collecting data for a small fraction of that. We've already learned most of what we can (or at least what we can justify the cost of a launch for) with those simpler missions. This new Europa mission is going to be big. Even in physical size it will dwarf those old transistorized tin pots. NASA calls missions like this "Flagship" class. They are few, far between, and generally bring in floods of new information. This mission is on the scale of Voyager and Cassini.
Even back in the Voyager days, when the rocketry and resources (developed in the lull between Apollo and Shuttle) to launch such a mission were newly available, close visits to any of the planets beyond Mars were completely unprecedented, and NASA was anxious get underway it took five years. Cassini was first proposed 15 years and approved I think 10 years before it launched. Now that there's minimal hurry and a lot of other things to share the annual budget with, so the timeline is more like that for Cassini. The taxpayers don't want to pay out more per year, and besides, Europa isn't expected to go anywhere in the meantime.
As mission complexity and cost grows, getting the most out of it becomes increasingly important. You can't achieve that with a generic bus because it limits the instrumentation you can hang on it. Instead you tailor the bus to the power, thermal, geometric, stabilization, and other needs of all this really expensive and fancy instrumentation. If you need a 3-axis stabilized, nuclear-powered spacecraft with a large contiguous cavity for a big telescope like Cassini, you can't make effective use of a solar-powered spacecraft bus designed to be spin stabilized and provide a mount for a radar and a long magnetometer boom like Juno.
Instrumentation is another thing. Back in the Mariner days, they were generally taking the best instruments currently coming out of the labs and figuring out how best to use them for the mission. Lately, it's been more typical to examine what you want to know, what technically should be possible, and do the research, development, design and testing of an instrument optimized for its mission. As a result, science package development is often a primary pacing and budgeting concern for exploration missions these days.
Lastly, those ten Mariner probes in ten years were being concurrently developed, not one after the other. I'm not sure how many missions NASA had active or in development at any given time back in the 60's and 70's, or how much money was devoted to them. Right now, however, I'm aware of 13 solar system exploration missions currently operating, and five or six more in development. I'm really not sure how many earth and deep space observing missions there are (Hubble, Spitzer, Chandra, GALEX, WMAP, OCO, JWST, etc). All of these consume (I count 60+ total on NASA's website) consume less than a quarter of NASA's budget. It's rather impressive in the grand scheme of things.