Well, the "HT" in the designation probably stands for "high traction." The HT-B truck was used on 3500 hp BB prototypes at a time when many railroads thought even 3000 hp needed six axles (witness the popularity of the SD40-2 versus SD45-2). So, at a guess, the new truck design offered higher adhesion. It might be interesting to compare it to Alco's Hi-Ad and to the MLW "ZWT" (zero weight transfer) and GE's FB-2 designs. My suspicion is that in each case the point at which tractive effort was transferred to the body of the locomotive was lower-- closer to axle level-- than in earlier designs used by the same builder (I know this is the case with Alco's Hi-Ad; I think GE's FB-2 is similar in principle to the Hi-Ad, substituting a more easily maintained stack of elastomeric and metal plates for the Alco design's coil springs), thereby reducing the weigh transfer that tends to unload the leading axle.
Why wasn't it adopted more widely? Technological advance in locomotives was painfully slow in that era: the main railroads were very conservative in what they would order. Something NEW, something that would involve NEW SPARE PARTS INVENTORY, even if clearly somewhat better than the existing (Blomberg with extra shock absorbers) design, would be passed up. So: maybe the HT-B turned out not to be very much better than the Blomberg, maybe it was more expensive, but I suspect it might have been the market flop it turned out to be even without drawbacks like these. (Compare: the HT-C truck introduced on six-axle Dash-2 designs was supposed to improve adhesion by reducing weight transfer as compared to the older Flexicoil truck. But one major railroad-- Conrail-- insisted on having its SD40-2 and even its initial orders of SD50 built with the old truck.)
(((Question I don't know the answer to: could HT-B and Blomberg trucks be interchanged, or did they require differences in the locomotive underframe? If the latter, there would be an additional motive for conservative motive power authorities in purchasing railroads to stick to the older design.)))