Ariane 5 was built with the need to launch the Hermes shuttle, hence why it is such a heavy lifter - and why it is the safest launcher, as it was meant to be human-rated. But then as very few satellites now are so heavy, they have to rely on this weird contraption to launch two satellites at once.
Launching only one per flight wouldn't be competitive even with reuse, as reuse wouldn't cut the launch by more than half. And to launch so many rockets, they would also probably need to expand their launchpad facilities by quite a lot.
What they are going for with Ariane 6 is a lighter launcher (also to replace the Soyuz medium launch) and, recently, a cluster of light, cheaper engines instead of the big expensive Ariane 5 one. Which is a good idea, as smaller engines are easier to build and you can make economies of scale - if your engines are absolutely safe. That's what Antares used with those superb Soviet engines made for the N1, but as we've seen, just one engine failure doom the entire rocket, multiplying the need for safety by many times.
Airbus is working on a system to bring the engines back, but it doesn't seem to be a priority.
What I don't get is why they don't go for continuous detonation engines for those new engines. Airbus and MBDA (the ones making the new small engines) were already working on those ten years ago, and it should be cheaper, simpler, lighter, more versatile and more efficient (though possibly louder, but who cares about that for an already ~200 dB rocket?) I guess the tech is still too young to be used in production in the near-future...
Unfortunately, info on continuous wave detonation engines and their current status seems difficult to find.