Winmodem configuration can be messy, because Winmodems use a range of memory addresses instead of a fixed memory address. Winmodems require a DLL file that loads into memory at startup, using some of Windows' resources. In part, the answer is cost. By omitting a few chips, manufacturers can sell Winmodems for twenty to fifty dollars less than traditional modems. In an era of thousand dollar PCs and handheld consumer devices, cost is an enormous issue.
Soft modems have some real benefits in small devices, such as laptop and hand held computers.
They weigh less than traditional modems. They take less space. They also consume less electricity, which makes them ideal for battery-powered devices. The upgradeability of soft modems is mixed. RPI modems aren't upgradeable to anything beyond a 14.4.