Original: Most observed white dwarfs have relatively high surface temperatures, between 8,000 K and 40,000 K/O. A white dwarf, though, spends more of its lifetime at cooler temperatures than at hotter temperatures, so we should expect that there are more cool white dwarfs than hot white dwarfs.
Simplified: Because white dwarfs are extremely small, it takes them a long time to cool down. Eventually, all white dwarfs will cool down into what is called a black dwarf.