Long range, wireless power transmission
There are no difficulties to overcome.  There is nothing to develop.  It is already here.  It's used all the time, you've certainly used it many times today.  It can cover distances of miles or more as well.  

It's called radio.  

Unfortunately, far-field power transmission spreads out in all directions and is absorbed or reflected by whatever it ends up hitting.  Some of it simply goes off into space, most of it just gets absorbed by the terrain and this causes a totally insignificant amount of heating of said terrain.  A very tiny amount can be absorbed by wireless power receiving stations, better known as antennas and radio receivers.  

The amount of power you can pick up is proportional to how big a slice of an imaginary 'sphere' around the transmitter your antenna can cover.  At any useful distance, even a massive antenna is going to still only represent a tiny fraction of the surface of this sphere.  The cold hard truth (which is backed by the highest authority - mathematical proofs, specifically about Euclidean geometry) is that radio is as good as it will ever get.  The idea of sending any substantial amount of power by traditional, electromagnetic means is fantasy, nothing more, nothing less.  

There are of course natural means that achieve this, such as the wireless power transmission that goes on for about half a day, every day.  The transmission station is the sun.  The electromagnetic waves are just extremely high frequency, but light is still part of that spectrum.  As you can clearly see, the light emanates from the sun in all directions, and the amount the Earth picks up is only as much as what hits its surface.  The fast majority of the sun's energy is hemorrhaged out into space, unused.  This is a much more visual and clear version of the exact same thing you are talking about.  Long range power transmission is terrible in every measurable way, and there is no way around that.  Even if you use reflectors, it will block power from being received in other directions, and also require aiming at a fixed location.  At which point it is little more than an extremely inefficient and wasteful version of wired power transmission.   

You could of course do things like use a very high power laser (or maser) with high spatial cohesion and point it at a receiver (like a solar panel optimized for this laser).  However, this is utterly inane.  Why would anyone ever do this? It would be significantly more expensive, much less efficient, unreliable (pretty much anything that gets in the way can scatter or simply absorb it), more dangerous, and has no tangible benefit or advantage. Just run electrical cable, it is all the things that this vaguely plausible means of wireless power transmission isn't, and those things are the only things that actually matter.  

That brings me to my final point, which is that even if it were possible to transmit large amounts of power wirelessly using far field radiation (which it isn't and we already know will never be), we wouldn't because we shouldn't.  There is no reason to.  Wires are reliable, relatively safe, and much more efficient than far field transmission can possibly be.  It's cheap, it's simple, and it works.  Wireless power transmission is unnecessary and inferior in every meaningful metric.  The only valid reason I can imagine anyone having for considering it is because that person finds it 'cool'.  Fortunately, civilization is built on what matters, not what a person or persons think is cool.  Which is why long-range wireless power transmission would not be pursued even if it was practical, which it isn't.  

Even short range wireless power transmission is completely unneeded and being marketed entirely on it's 'coolness'.  I find it wasteful and stupid, which is exactly what it is.  With a cord, I can use a device in my hands or any location in range of the cord I please.  The efficiency of a cord that short is well above 99%.  And it uses a connector that, save for a few unlucky outliers, will not wear out before some other critical part, like the battery, wears out.  The connector is not the wear-out mechanism.  

A wireless charger requires the device be correctly positioned and very close to the charging pad.  I can no longer use it easily, the battery still wears out as fast, and I am needlessly wasting energy for the most asinine reason.  Because it seems 'cool'. Oh, and wireless chargers work via near field magnetic coupling, so their existence and behavior is completely irrelevant to long range wireless transmission.  They work via induction, a strictly near field effect.  They also emit far field radiation.  And its completely useless to your phone because no meaningful amount of energy can be received by it.  

So unless you call the truths we can show with actual proofs regarding surface area (which ultimately govern how much power can be harvested wirelessly - since you only get as much solar power as there is solar panel to shine on) 'difficulties' (which would be wrong - they aren't difficulties. They are absolute truths - mathematical proofs are the only things that we know are true), then your question should be 'why is wireless power transmission impractical?'.  There is no obstacle to over come.  Its used to its maximal and most useful extent already, and to great benefit.  I certainly enjoy radio, cell phones, wireless internet.  But the power levels possible are restricted to levels only useful for communication, and attempting more would be unethically wasteful.  

I know none of this was probably what you wanted to hear, but what we hope is true and what is true are two things that need not be aligned, and often aren't. I understand the appeal of the idea, and let's be honest, it is a cool idea.  But then again, so is Santa Claus.