Why were my photos cropped when I ordered 6x4 prints?

I'm assuming you probably used a compact type camera with an image shape of 4:3 (long side of image is 4/3 longer than short side, which describes a "shape").

But a 6x4 print is the shape 3:2... simply not the same shape. So not all of the image will fit on the paper. Images generally have to be cropped first to fit the desired paper shape. And most print sizes have a shape different from each other, and probably none are the shape of the image (before it is cropped).

6x4 inches - 3:2 shape
5x7 inches - 5:7 shape
8x10 inches - 4:5 shape

compact type cameras - 4:3 shape
DSRL cameras - 3:2 shape
Camcorders - 16:9 shape.

The print lab operators could watch and manually position your image on the paper to avoid cutting off heads, but for only a few cents each, normally today, the machines are on full automatic, and the machines don't know or care about heads. It is our job to do today, first.

Many online printing services offer a web page crop tool to aid this crop effort.
The larger walk-in shops should have a machine with a screen to aid this crop effort.
Or my site has a description of how to do this at home: http://www.scantips.com/lights/resize.html