"Nerfed" isn't really the right word to describe what happened to Hidden Power in Generation 6. Changed is a more applicable term; while Hidden Power is definitely less useful now due to its lower potential base power (from 70 down to 60), it is significantly more convenient and is equally usable on any specific Pokemon compared to another. This is notable, because this wasn't the case in Generation 5 and prior.
Before X and Y, Hidden Power's type was not the only thing that was variable -- so was its base power. While Hidden Power could cap out at 70 base power, for most Pokemon this would not be possible. Depending on the Pokemon's IVs, the move could have anywhere from 30 to 70 base power, so unless the Pokemon in question has the high IVs that turn a strong Hidden Power, the move is weak and useless. This was changed in Generation 6 so that the move's base power would always be 60, regardless of the Pokemon's IVs.
To me, this is not a nerf, but merely a compromise between balancing the move for non-competitive players and maintaining its viability for those who are competitive players. The move's mean base power, given a range of 30 to 70, is 50 so it may be have been an expectation that the move's power would become 50, as it's right in the middle. However, the power is instead 60 as of X and Y, so if anything, I'd say the move was made better overall. Competitively, however, it's weaker and less useful, but still finds frequent use on some Pokemon.
On a related note, what makes you say that nobody used the move? In competitive play at least, the move was extremely popular on a lot of special attackers as it gave them access to coverage they may not get access to otherwise. For example, Hidden Power Ice was -- and still is -- extremely common on Electric type Pokemon as it offers them coverage of Ground and Dragon type Pokemon that resist their STAB attacks. Even though it's slightly weaker, Hidden Power is still a fairly common move, especially Fire and Ice type variants. Ice is a severe weakness for many Dragon type Pokemon that have a Flying or Ground dual typing, so Hidden Power Ice is naturally a favourable option. Similarly, Hidden Power Fire is useful to break walls such as Ferrothorn or Forretress that are 4x weak to the move.
