There are a few special things about Pluto, as compared to the other dwarf planets in the [Kuiper belt][1]. These include:

1. It is the largest dwarf planet known, by diameter. (Note, this was only determined after the [New Horizon's flyby][2])
2. It orbits relatively close to the Sun, at times it is even closer than [Neptune][3]!
3. It has the largest satellite system of any non-gas giant in the solar system.
4. It has an atmosphere. It seems unlikely that any other dwarf planet has an atmosphere that has been discovered thus far.
5. It is the only "double planet", where the planetary satellite system's barycenter outside of the system's main planet!

Okay, so why else was it discovered early? Here's a few reasons:

1. [Telescopes][4] didn't grow much in size for some time around the discovery of Pluto.
2. Discovering new objects so far out takes a lot of time. Someone really has to be looking for such objects, or else have a dedicated computer program to do so. In fact, often objects are discovered to be in pictures from long ago, such as [Orcus][5], discovered in 2004, but pictures were taken of it in 1951.
3. Computers didn't come to be popular for such purposes until the 1990s, which is why the number of such objects discovered increased dramatically at that time.
4. Pluto is [quite a bit brighter][6] than the other dwarf planets, [Ceres][7] excluded.
5. Astronomers were looking for a new planet in the outer Solar System when Pluto was discovered. Pluto was not the planet they were looking for, still, it was there, largely by coincidence.
6. The 1990s saw a dramatic improvement in telescopes, namely in that the mirror could be moved in a way to correct for atmospheric issues, called [adaptive optics][8].

  [1]: https://en.wikipedia.org/wiki/Kuiper_belt
  [2]: http://news.sciencemag.org/space/2015/07/pluto-confirmed-largest-object-kuiper-belt
  [3]: https://en.wikipedia.org/wiki/Neptune
  [4]: https://en.wikipedia.org/wiki/List_of_largest_optical_telescopes_historically
  [5]: http://phys.org/news/2015-08-dwarf-planet-orcus.html
  [6]: https://en.wikipedia.org/wiki/Apparent_magnitude
  [7]: https://en.wikipedia.org/wiki/Ceres_%28dwarf_planet%29
  [8]: https://en.wikipedia.org/wiki/Adaptive_optics