Why is Sky so hard to crack....
The simple answer is their years of experience with piracy, huge finances and good insight.
Technical reasons include a unique special made hardware within the smartcard, known as an ASIC; anti-layer removal; anti-probe; anti-glitching/unlooping/voltage attacking; own hardware manufacture; high security at manufacture level; high security at card pre-issue level; high level encryption of data; technical math formulas; not giving licence for 3rd party hardware; not being compatible with other systems; and probably more that i've forgot at the moment lol.
They also heavily police the web on an 'Active' basis rather than a 'Passive' basis. Teams of paid profesional hackers, fake websites, spies, mafia connected recruits are all known to have 'unofficial' connections in their history, it is a very publically hidden dark business that they alledgedly invest in.
You need to break down each reason to learn more about the security, but basically its a unique system that they have developed from years of piracy experience over almost 2 decades. Nothing is impossible and its been cracked many times before, its just that as soon as anything is released money gets it closed.
Hacking anything is about hitting the weak points. Satellite tv is hit elsewhere with easier encryptions, its hit with 'sharing' and sky is hit abroad where they have less legal backing. So there really isnt a massive need for a sky hack imho..... which is a final reason perhaps?