If you follow the link to the "analysis" document it summarizes their rationale. It looks to me that the summary is "in reality we're not going to be sueing any

If you follow the link to the "analysis" document it summarizes their rationale. It looks to me that the summary is "in reality we're not going to be sueing anybody, and we think the chances of a contributor sueing us are negligible, so while a CLA provides some extra legal protection/capability in theory, in practice we don't expect to use that and so all we get is the downside of extra bureaucracy and a barrier to contributors". Different GPL projects vary in their attitude to the importance of enforcement -- the FSF cares a lot about retaining that option for their projects, but the Linux kernel community doesn't really care much. (And some kinds of enforcement, especially the most common "work with the violator to help them become compliant without taking legal action" kinds, work fine even if the copyright is not all held by a single entity.)