Yes, cost may be an important factor, but it is not just that. Slicer and commercial clinical applications are playing in different fields.
For research, prototyping, and education purposes Slicer is much better, due to its openness, extensibility, comprehensive feature set, and large, very helpful and knowledgeable user/developer community.
For routine clinical tasks, you are required to use regulatory-approved software, with simpler, locked-down user interface and feature set. Mimics and other commercial software address this need directly. In contrast, clinical use of Slicer is the user’s own responsibility (we need obtain ethics/institutional review board approval for each study) and often user interface customization is needed for ease of use.