My take is that this is a combination of:
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B1 coming out at a time where the pickings for the genre were otherwise scarce; and
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B1 being the best of the series by far by nearly any metric you’d care to use. It features a much better villain (and, arguably, the only one so far worthy of that title), the writing is better and tighter (even if Sera’s use of “insular” there made my eye twitch every single time I read it), and the words that would otherwise be padding are instead used to worldbuild (which, understandably, B1 has to do more than its sequels).
Collapsed for brevity
I wouldn’t even put B1 in the same GENRE as the other two; it has a much more serious tone and leans heavy at parts into horror. As an example, in B1 a teenager has his throat ripped out and it is done in an especially brutal and unnecessary manner for no other reason than the villain is out to send a message. When you tell his mother, she breaks down in tears. She goes by the police station later to ask about news on the case and she breaks down again there. Bad things have happened and people act like bad things have happened. It’s great. In B2 a bunch of people get infected with a magical incapacitating, fatal disease with no known cure and in B3 there’s a series of kidnappings. I’m going to let you guess how often NPCs other than your team bring either of those up during the books.
Like, if you gave me scenes of B1 and B3 with the serial numbers filed off and told me they were written by the same person, let alone were part of the same series, this would be the look on my face: 
Anyway, I hear Sera’s rewriting B1 to be more in tone and writing style with the rest of the series, so ooops.