I am not a historian but I do have a degree in natural resources and have worked with foresters, arborists, planting specialists, and ecologists.

The tree is a poor ornamental for several of the reasons you listed but also dangerous for others as well. It is considered invasive by many US states and there are efforts underway in some states to eradicate it. Here in the southeastern US, it springs up in vacant fields, along roadways, and elsewhere at ease. The species is a nuisance and it will be very difficult to rid our country of the species now that it has become established in the wild as an invasive. Additionally, it is considered to be weak and brittle and consistently breaks and splits away at the trunk or major limbs.

So what's the answer to your question, why was it planted in ubiquity during the second half of the last century, and the reason why it is has lost favor as an ornamental in modern times?

Let's start with some history of the species. Pyrus calleryana, more commonly known as the Callary Pear, is a tree species in the Rosaceae family (includes many fruit tree species--apple, pears, plums, cherries strawberries, etc.). The species is native to China and Vietnam. The cultivar "Bradford" is only one cultivar of this species. A cultivar is a type of plant that has been bred for a desired trait. Examples include disease resistance, floral color or size, improved yield or flavor, or plant form (size/shape/etc.). Other callory pear cultivars include Chanticleer, Rancho, Avery Park, Aristocrat, Redspirea, Autumn Blazer, among others. These other cultivars were chosen to try and create more upright trees and to try and avoid the Bradford's poor branch structure and propensity to break.

The Bradford cultivar was the first one created around 1960. The seed for the original cultivar was purchased in Nanking, China in 1919 (SPI 47261) and planted at the USDA Plant Introduction Station in Glenn Dale, MD (Santamour and McArdle 1983). It was further promoted by Ladybird Johnson by her planting a tree in downtown Washington D.C. in 1966 (Popkin 2016).

The Bradford cultivar was used by landscapers in the 1960s through 1980s because of its many favorable attributes. The most important of these to landscapers was the fact that the Bradford cultivar was considered to be self-incompatible (due to being sterile), or unable to produce fertile seeds when self pollinated. In other words, the cultivar can't spread by breeding with other Bradford cultivars. This means that the Bradford cultivar itself isn't invasive. Callary pears become invasive due to the crossbreeding between different cultivars within 300ft of each other. The resulting wild individuals can then interbreed among themselves and spread at a rapid pace. Additionally, if the rootstock of a Bradford is allowed to sprout (knicked during mowing for example) and the sprout from the root flowers, the rootstock can cross-pollinate the mainstem of the Bradford and produce viable seeds. There is a lab in Oregon studying the cross-pollination and it appears through preliminary study that any of the cultivars planted together can cross-pollinate (so not isolated to Bradford alone). There are multiple reasons different cultivars were created, one reason was to create a tree that wouldn't split and fall apart under wind/snow events.

Other favorable attributes of the Bradford cultivar include pretty white flowers that flower in early spring; it grows upright and symmetrical in shape; it can grow in urban areas along streets in poor soils (wet, dry, acidic, or alkaline); it is fairly resistant to disease and insects; it has pretty fall colors to go along with their spring coloring; and it doesn't drop messy fruit like mulberries or crabapples due to its sterility; and the sterile cultivar doesn't have thorns.

Since this is a history sub, I feel like I need a paragraph describing the history of how the Callary pear and Bradford cultivar made its way from China to the USDA station in Maryland. At the turn of the 20th century, Northern CA and Southern OR were the centers of pear production (common or European pear) in the US but were susceptible to a fungal disease called fire blight. A plant scientist in OR named Frank Reimer worked on fire blight control and attempted to graft the Callary pear to the European pear in order to potentially make a hybrid that was resistant to fire blight. The Callary pear has a much smaller fruit that is inedible but is used for tea in China. Reimer employed botanist David Fairchild, of Japanese cherry blossom in DC fame, to locate seeds in China that could be used towards his research. Fairchild employed Frank Meyer, of Meyer lemon fame, who traveled to China in 1916 in search of a super-pear. Meyer ultimately perished on a steamer headed for Shanghai on the Yangtze under questionable circumstances in 1918 (drowned in the river at the age of 42). Horticulturist John Creech headed the USDA station in Maryland in the 50s and he latched onto a single specimen of Callary pear growing in the station that was planted in 1919 and originated from Nanking (the seed for this tree did not originate from Meyer but an expedition sent by Fairchild after Meyer perished). Creech is personally responsible for the spread of the Bradford cultivar as this particular tree was used to spread the cultivar across the US. He named the cultivar after the previous head of the USDA station, F.C. Bradford. He tested the cultivar in nearby University Park, a treeless D.C. suburb known for its poor soil, in 1954. He officially released the Bradford to the nursery trade in 1960 and the spread of the Bradford took off from there. Creech and other experts of the time ignored various clues to its invasiveness and the tree was allowed to spread until a journal article in 1977 written by Ackerman pointed out the potential for the tree to become invasive due to cross-breeding with a more upright Callary cultivar called Whitehouse. What is the most interesting fact for me is that the callary isn't considered invasive in its native China. Therefore, all of the historical figures above played a hand in personally creating an invasive monster that is ubiquitous today. Creech never publicly regretted his decision to introduce Bradford to the world. The majority of the details here were derived from a fabulous Washington Post article from September 2018 written by Adrian Higgins.

To speak to your specific question regarding the flower smell, the smell is a mix of trimethylalamine (fishy smell) and dimethylalamine, and possibly a few additional chemicals. Some consider the smell to be similar to semen, however, the smell is unique to Bradfords and is a mix of multiple volatile amines, which are molecularly similar to ammonia. The smell we attribute to semen is due to the presence of ammonia.

The purpose of the fragrance is not fully known but is assumed to be a signal to pollinators. So, who pollinates the species? According to Barbara Fair, an associate professor and landscape extension specialist at NC State (my alma mater!), flies are the primary pollinators. Bees will also pollinate the flowers but flies are the main pollinators. Flies typically like bad smelling things so this makes sense. Bees tend to prefer sweeter smelling flowers but can be opportunistic at times. She personally thinks the trees smell like baby poop but indicated it is of least concern due to the invasive nature of the cultivar.

We will never quite know why Creech decided to choose this particular tree given the smell associated with the Bradford cultivar, however, he likely ignored this minor factor in considering the positive ornamental nature of the tree and the ability to grow it practically anywhere. Additionally, the original tree may not have had as strong a smell as the trees we smell today. That is one of the downfalls of trying to play "God mode" with plants, they are inevitably difficult to control and consistently perform in unexpected ways they sometimes leads to poor outcomes.

Edit: Made a few grammatical edits.