That's a shame! I can imagine your frustration.
I highly recommend you create + use an application tracker moving forward. In this you should track:
- office(s) applying to
- recruiter name/contact info
- application deadline
- application status
- interview date
In terms of why they had an early application deadline, this is twofold:
1) To "eliminate" lesser candidates. Candidates who are on top of their game, well organized, and truly care about BCG would be far more likely to apply on time. This ensures a higher % of the pool of applicants is a good fit for BCG (this is a market for lemons, so signalling is important)
2) To get ahead of competitors. If they can interview and make offers before their competition, they may "force" candidates to accept their offer before they know results from another company they are more interested in.
Like in life (and consulting!) learn from your mistakes - get organized today!