There unfortunately isn't one answer as to why the corps is smaller this year. It truly fits into the "stuff happens" category. As I talked with fellow hornline members at the end of last season, the reasons why people weren't planning on marching this season were all over the board. Some were getting married, some were starting new careers, some were going to DCI, some were starting families, some just wanted a summer off to relax and then some didn't want to march because all of their friends has decided not to march, and undoubtedly there were likely some that didn't have fun or had issues with the staff. My reason? I had marched 19 years and wanted to see what "normal" people do with their summers.
MBI has historically (in the last 10 years) had a good sized contingent of long-term vets the come back year after year so recruiting hasn't been a large focus. You can usually pull in a few bodies over the course of the season, whether HS kid, veteran or jr corps kid when they get off tour. This year however, it ended up being a perfect storm of vets not returning that I think even the best skilled recruiting corps would struggle with. I think regardless of score or placement at finals, the corps itself will emerge a stronger entity after this season through bolstered recruiting practices, improved alumni relations and a little introspection on what the corps wants itself to be.
Note: I do not speak for the corps. Any observations are my own.
From a earleary post on MB