For me, it was the sonic qualities of the classic 3B, the generation before the NRB, 19“ flat black font panel with handles…
I was jobbing in a HiFi store in the afternoons after school and we had a new MB-Quart 3200 S 4-way Tower speaker with a 12“ woofer in out High-End Studio. The supposedly best power amp we had in there was a T+A Pusar A3000M, which had stellar reviews from the German magazines and was rated „Absolute Spitzenklasse“, i.e. highest possible rating.
Driving the MB-Quart with the A3000M, the music sounded lifeless. I checked all the connections, as I suspected that I might had mixed up polarity. I even considered that the speakers might be broken… then I replaced the T+A with the Bryston 3B, which the representative of the German distributor had left at our disposal for testing. With the 3B in the signal chain, I hardly recognized the MB-Quart speakers anymore as there was life in the music, it was literally like night and day! The back then 2700 DM (Deutsche Mark, appr. 1350 EUR) 3B was so much more musical than the almost twice as expensive T+A power amplifier. From an engineering standpoint, the T+A was a marble, but in combination with those MB-Quart towers it did not stand a chance.
Well needless to say, that very 3B landed in my private system, powering my Sonus Faber Minima FM II. Since then, I exclusively used Bryston power amplifiers in my system. The 20 year warranty made it safe to acquire used Bryston equipment, so I was able to afford a number of them.
Long story short: Why Bryston? Sonic qualities and (back then) incredible value.
That being said, the sonic qualities of the current product lineup certainly are great. As cost and in consequence prices have gone up considerably in the past 30 years, the value question is much harder to answer. We have been at this point before in the new product discussion. 2700 DM was not cheap back then, but the accessibility of Bryston gear most probably has not improved over time. This probably is the point that Bryston need to address.
Edit: adjusted for inflation, the 2700 DM from the late 80s translate into appr. 2500 EUR. A 3B cubed is listed at 6827 EUR on the German distributor‘s website. Certainly, a current 3B cubed is way better and powerful than the late 80s 3B, but it is also more than 2.5 times more expensive, adjusted for inflation! That‘s the point I wanted to make.