The concept of memorial parks was born in the early part of the 20th century. Until that time cemeteries were seen more as public parks and were designed to be beautiful, filled with ornamental trees and flowers, only upright granite or marble monuments, lots of statuary or other "features", winding roads, slopes, etc. Some decided this was too much maintenance and designed what were called memorial parks. The concept was low-maintenance, sparsely landscaped with only flat bronze or stone grave-markers, for ease of maintenance and mowing. Many memorial parks were built and continue to be the norm for most newer cemeteries (i.e. those developed after 1920 or so). This is changing somewhat as the baby-boom generation is very much insistent on personalized funerals and burials. Some cemeteries are starting to open new sections that incorporate upright monuments again.
There is some worthwhile background material on the memorial park concept located in iccfa.com archives tagged with memorial park and cemetery esthetics