Eh, that's overstating the case. There's clearly some aesthetics that are more appealing to more people but for many architectural movements in particular the reason that they look that way is for the way that specific ideological reasons interacted with material constraints and the intended message. Brutalism in particular was intended to be cheap and honest; given the constraints many of these buildings were designed under, it makes sense. There are some quite appealing brutalist buildings; a core tenet of the style was integrating the buildings into the natural landscape, in contrast to the artificial styles that had previously been popular. The post-war shortages limited the available materials, shaping the constraints they were operating under. Raw concrete was honest, cheap, and was allowed to weather naturally.

There's a lot of ugly brutalist buildings, but there's a lot of ugly buildings in every style. At lot of them look cheap because they were supposed to be cheap; to a certain extent looking inexpensive was intended. In some cases the hostile nature of the institutional building was part of the point, conveying strength unstead of offering a pleasant experience, but there's also some quite pleasant brutalist buildings that have a lot of nature integrated into the design.