I think in the end its the quality of product, its association with the brand name, very much like other products. However, unlike consumer items, T&M gear have useful lives measured in decades, so a product with such a reputation is very difficult to overcome.
The 87V Fluke DMM has been around since the late 1980s, the Agilent 34401a from the early 1990s, 3458a from 1989 [ enduring while most of the 8.5 digit Metrology DMMs have folded]. I think oscilloscopes are not in the same position because analog, where Tek was the unchallenged leader, has all but gone and the models have a shorter half life.
Likewise, among the second echelon brands, Global Specialties has an established reputation in low cost pulse generators: the 4001 and the 4010 are still being made, the casing and color schema are different, but the design is over 20 years old.
Sanwa in Japan still makes the YX360, an analog VOM, which was designed in the late 1970s. I rarely 'see anyone in the west use it, but it has a popular following in Asia and is part of Sanwa's mystique and reputation in DMM.
Simpson, once the king of VOM, still makes its signature 206 to this day, this was the analog VOM for accuracy and durability, designed in the 1930s! I'm not sure which industry still uses it.
Finally, the modern versions of these models have incremental improvements, but they are more similar to their originals than different.