It's literally because BQ is a tough target time. Even if you don't run Boston it's a mark you're in the top X% (X is a bit hard to calculate). So it's a status symbol. Just like 'Ivy League' or "D1 sport".
Similarly in the UK, it's the London qualifying time known as Good For Age or the more challenging 'Championship Place'.
And it's self-fulfilling. You get the time so you chose to run because you have it which keeps the time hard for others.
The race itself is, I'm told, a pain-in-the-ass because of the logistics but also the profile - despite being net-downhill it's got a nasty hill at mile 20. Plus with the race route being pretty much "26 miles straight, then hook a right", if there's a headwind, there's a headwind for 26 miles. If there's driving rain, it's in your face for 26 miles (see 2018's race).
(As an aside, there's a few tricks for the Abbot Majors to get places [aside from just buying one of the expensive guaranteed tour company places or being an elite runner])
