Because they don’t push updates as quickly, which reduces the chances of something slipping through, be it their merit or not. This comes at the expense that it sometimes breaks dependencies and still has close to zero real benefits:

1. You are better off simply using snapshots. Then you don’t depend on the testing of either party.

2. Even if the Manjaro devs do to find bugs, they could have found them in Arch Testing as well, which benefits everyone.

I stand by my point that the update strategy is not a feature.