Because in order to have 2 of something we must firstly have 1 of it. and to have 9 of them, we must first have 8, and so on down to 1.
You cannot have 9 houses on a street without also having 8 - but you can have 8 without having 9. If all the streets in a city were to be ranked by numbers-of-houses, then there would be more with 7 than with 8 and more with 8 than with 9 - it's the same for any counting operation.
Have a search for "Benford's Law" - which deals with the probabilty with which certain numbers will be the leading digit in a set of numbers.
