Because having an integrated modem inside a router is:
1) cheaper
2) allows a smaller box (in stead of 2 small boxes)
3) is easier to manage
4) draws less power (thus produces less heat and will make less noise)
5) requires only one IP address "on the outside" (if you want to access the modem too, for management purposes, like upgrading the firmware).
6) simpler network design (one less bridge in the picture).
In my country, all ISPs supply their customers with routers with integrated dsl-modem, or integrated cable-modem. I believe that is the case in more european countries. I only hear people talk about separate modems and routers in the US. I prefer an integrated one (as long as it is good, ofc). I wonder how long it will take for the US to follow.