There are a few reasons.

• The Builder plugins (S_Effect and S_Transition) rely on features that are not provided by all hosts, such as the ability to hide/show parameters. We are working with host vendors to provided Builder support where possible.
• Some hosts have technical limitations that either restrict Builder's usefulness or prevent it from ever working; some hosts use a "grid" layout for parameters for example, or can only support a certain number of parameters.
• Some hosts are already node-based and have always allowed you to do what Builder does (e.g., Nuke and Flame)