I believe it is quite clear what has happened to you. I have also seen this with many candidates who saw themselves as "well prepared" after having practiced based on what is commonly available in terms of case literature. However, there is a very important thing that most people are unfortunately completely unaware of: the structures in the most popular casebooks are mostly arbitrary and suffer from a fundamental lack of rigor. They are usually written by authors who have not been long enough in MBB firms to have acquired the skill of logic-based top-down structuring (which you usually only start using systematically as an experienced project leader, once you start designing (not creating!) proposal documents for clients). So here are a couple of fundamental points to understand:
- Creating a strong structure does NOT mean to just tell the interviewer which areas you want to look into. This is not a structure! It is just a bucket list. Even if it contains dozens and dozens of elements!
- In order for a structure to be strong, it has to be a logic - the logic according to which you will answer the question at hand. It is ideally rooted in a top-down disaggregation of the criterion by which the client objective is met. This client objective underlies the core question.
- The disaggregation is best done with a driver tree, which allows you to identify the conceptual drivers and sub-drivers of your focus metric (thereby you create a (mostly) quantifiable operationalization of the client's objective).
- Only AFTER this is completed (i.e., the logic is established), then qualitative elements (such as consumer demand, market structure, company operations, etc. --> the "buckets") can be outlined and mapped to the sub-branches of your driver tree!
This is how you create an integrated approach which is focused, rigorous and does not rely on industry knowledge, gut feeling or just luck - contrary to the typical "bucket frameworks" that you can find in most canonical case books. These frameworks usually have a lot of good content, but lack the most important part - the inherent logic of what you are testing for! They emerge from a quite immature way of thinking, similar to what people are conditioned for in most schools or even universities (aka, learning things by heart, which goes against everything that MBB firms are testing for and which explains why such bucket lists are seen as an indicator for inherently weak thinking).
P.S.: It takes some time to learn and internalize this. But speed should not be your focus when learning this! Even if you need five minutes or more at the beginning, this is totally fine! It is like learning an instrument - you first have to play 10x slower! Otherwise you will NEVER learn in properly!