Ie, they tried to make buildings that looked Brutalist, instead of trying to design them to be functional and eschewing (or not paying as much attention to) aesthetics, which (in my uninformed opinion) is what Brutalism, as a movement, was about.
In my case it was that the architect (who worked in early 1960's) could foresee classrooms with tables, but could not foresee that the classroom tables would have computers connected to a network (as they had in 1980's).
Building structures with exposed structural materials means there's no space for hidden infrastructure like cabling and power points. Building structures out of concrete in particular is antithetical to modern technology - no one could have seen WiFi coming, but it's a major hurdle in all-concrete structures.
The shift to power outlets and internet connectivity is a major aspect of usability; it's a shame that so much of Brutalism is in colleges and transit spaces where people urgently want those things.
Source: experiences trying to keep computer science classes in a university building designed by famous functionalist architect Alvar Aalto (university now named after him).