It's hard not because the engineering is fundamentally difficult (it's challenging but compared to a lot of engineering problems not terribly so) but rather because it's very unforgiving and it takes a great deal of foresight and risk management / business savvy to do what SpaceX has done efficiently and cost effectively.
Look at a couple of the issues they've faced and managed to deftly conquer. Take vehicle design, for example. If you design a purely expendable launcher you tend to want to minimize engine count and concentrate on 2nd stage performance, which leads to a design with an expensive 2nd stage and a cheaper 1st stage with very little throttle authority (due to using a small number of engines). That makes reuse very difficult and not very worthwhile because you'd need to either try to reuse the 2nd stage (a much more challenging task) or to heavily modify the 1st stage, which would then only save the cheapest component of the launcher. So if you want to concentrate on reuse you need a vehicle like the F9, with lots of 1st stage engines, and most of the cost in the 1st stage.
But even then you need to figure out how to run your business. It would be easier to envision the launcher with reuse an integral part of the design from day 1, which would lead you towards testing the reusability aspects of the design prior to the first launch. But that would be tremendously expensive and would make it more difficult to keep a company running while that was going on. SpaceX has been able to provide numerous orbital launches with their rocket and earn a tremendous amount of revenue before it is capable of actually being operated in a reusable fashion. A common engineering dictum is "you ain't gonna need it", to avoid introducing unnecessary design elements that aren't strictly immediately necessary. But without designing the Falcon 9 with reuse in mind from day 1 they would have had a much more difficult time in developing the reusable aspects. Note that SpaceX has made use of hundreds of millions of dollars of flight hardware in their various landing tests, all of which they not only didn't have to pay for out of pocket but actually earned a profit for. That's smart business. Meanwhile, for all of the launches where they haven't attempted landings they've added operational experience and real-world testing of their overall vehicle design and components.
Big, bureaucratic companies that are bringing in billions a year in revenues (like ULA or Arianespace) don't tend to engage in behavior like this, because they don't have the hunger, they don't have the boldness, they don't have the coherent foresight. What SpaceX has done is something that a lot of other companies are going to replicate, likely fairly successfully, once it's been proven. But getting to that point has required not just engineering acumen but enormous risk management skills and business savvy. They'll have a significant market advantage due to their head start for quite some time, and hopefully they will retain enough boldness and savvy to continue moving ahead even as others attempt to catch up.