Selection of a suitable lifecycle for the project/product is more of a technical issue. Then why is it covered in Integrated Project Management (IPM) rather than Technical Solution (TS)?
Selecting a suitable lifecycle is a function of the project requirements and the application domain. The selection is something that should be performed when planning the project activities, not while performing the actual software engineering activities. Selecting a lifecycle model and defining the project’s lifecycle phases are necessary for planning and estimating the project. That is why lifecycles appear in Project Planning (SP 1.3) and Integrated Project Management (sub-practice 1 of SP 1.1) . The selection of the lifecycle is a project management activity.
From another perspective, the purpose of Technical Solution is to design, develop, and implement solutions to requirements. And selecting an appropriate lifecycle model is an activity that needs to be done before you can design, develop, and implement a solution to the requirements.