Were there no A fares left? A "B" fare is usually more expensive than an "A" fare which would have put you right into First.
Plus, according to
Delta.com, there could have been other DMs who booked a B fare before you did.
2Clearance starts with B class and continues in descending order. If Medallion level and fare class are equal, time of ticketing will be the tie breaker.
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