When people speak they often try to use the fewest words or syllables.
When we say the years we try to use the expression that has the fewest syllables:
1500 = one thousand five hundred = 6 syllables
1500 = fifteen hundred = 4 syllables
2000 = twenty hundred = 4 syllables
2000 = two thousand = 3 syllables
1957 = one thousand and fifty seven = 8 syllables
1957 = nineteen fifty seven = 6 syllables
And, that's the answer... :)