Fashions in architecture change as in other areas of life.
Victorian slums were never designed to be attractive rather they were designed to provide third world standard housing for poor people and to make a profit for the building companies. Slum clearance was and is universally considered a good thing.
Other Victorian buildings were better than the slums and many are highly valued today but round about the 1950’s and the 1960’s Victorian architecture was out of fashion. Victorian buildings were considered ugly and frequently actually looked worse than they needed to look because they were badly maintained, soot and grime had not been cleaned off. There buildings were called Victorian monstrosities, some people complained when Victorian buildings that had been valued in their time were proposed for demolition but many were taken down despite complaints.
In the late 20th Century Victorian buildings came back into fashion, in Birmingham City Centre, for example the surviving ornate Victorian buildings are likely to be listed and Birmingham at last feels the city can be proud of its heritage. There is sometimes an economic cost to keeping the listed Victorian buildings for example the Birmingham road ironically named, “Broad Street” is too narrow for modern traffic and cannot easily be widened. Also buildings designed for 19th Century use are inefficient for modern use. There is also a clear economic cost to demolishing Victorian buildings that were built to last for centuries and replacing them with less solidly built buildings that may be replaced yet again within a lifetime.