In the book Pox Americana, historian Elizabeth Fenn tracks a smallpox pandemic that spread throughout most of North America into Central and parts of South America from 1777-1785. (Epidemiologists define pandemic as an outbreak of disease across a country or the world - it doesn't necessarily have to have a global spread). As for other pandemics started in the western hemisphere, it is certainly possible, though tracking the patient zero of a disease outbreak was difficult before epidemiology became a fully formed field in the 1950s (and is still difficult today).

To address your second question, a combination of factors, including massive global movement of people during the war, lack of effective control measures, and the particular strain of influenza virus involved perfectly set the stage for a pandemic.

Let's start with the human activity side of things. The flu likely emerged in late February 1918 in Haskell, Kansas where pigs and domestic fowl were kept in close proximity, which provided the perfect setting for viral exchange and recombination. The virus jumped the species barrier and underwent human to human transmission, causing a local epidemic late in February. A few of the victims of this early epidemic were soldiers on leave. They returned back to Camp Funston in early March without realizing they had picked up an infection. Upon arrival they began showing symptoms of influenza and were taken to the base clinic. The camp at this time was characterized by overcrowding to make room for an influx of soldiers, many of whom were to be sent to France shortly. This meant close contact with many naive (in this case meaning previously unexposed to this strain of flu) soldiers, an essential criterion for effective transmission of flu, which is spread via respiratory droplets (coughing, sneezing, etc.). The flu spread through this camp like wildfire. If no one was allowed to leave the camp, the flu could have burned itself out there. Instead, wartime movement did not cease, and soldiers were transferred from Camp Funston, where the flu was raging, to other US Army bases and, more importantly, to France. By May 1918, influenza had achieved a global distribution. This is what is referred to as the first wave of the 1918 influenza pandemic. Cases of the flu during this time carried a much lower mortality rate than was seen later, but the rapidity with which it spread through populations set off alarm bells in many physicians and public health professionals. This is also when the influenza picked up its name - "The Spanish Flu". Unlike several other countries which were massively hit by flu during this time, Spain was not engaged in war and so was not censoring reports of flu. Countries such as France and the US kept a close lid on the extent to which they were impacted by flu, so as not to appear weak. The first wave of the pandemic seemed to burn out around August, and the world breathed a collective sigh of relief. But this was premature, as the virus had only gone underground and would emerge in just a couple of months with a fury never before seen.

Late in September, reports of a deathly influenza in US military camps started pouring into US intelligence. Soldiers lying ill with influenza in camp clinics were dying within 24 hours of admittance. Most disturbingly, this was not confined to one camp or even to one country. Across the world young, otherwise healthy soldiers in the prime of their lives were dying within twenty-four hours of admittance to a camp hospital or clinic. Toe tags were being placed on boys who doctors and nurses knew were in their last hours of life. Army hospitals were so overcrowded during this time that newly admitted men laid on the floor next to a bed occupied by another man close to death. Something had happened to make the virus more lethal. As it is transmitted by respiratory droplets, influenza is most effectively transmitted when in close proximity with many other people. Most of the time, this means that you have to feel well enough to walk around to expose people. During the 1918 flu, camp and war conditions did the work for the flu virus. You could be sick as a dog and still expose a ton of people thanks to the overcrowded conditions in a camp. This is one way scientists believe the influenza virus was selected to be more virulent, more lethal.

Despite the urging of many public health officials, quarantine was not imposed and troop movement was not limited by military officials. The war was seen as more important than controlling the flu, never mind that the flu was picking off thousands of able-bodied troops. In October, the flu made its public appearance, devastating cities such as Philadelphia and Boston. Though flu was raging, wartime parades and rallies were not canceled so as to keep morale high. These crowded events provided a perfect opportunity for the flu to be rapidly transmitted. By the time that public health officials imposed any sort of control, it was basically useless. The face masks worn by so many didn't prevent flu transmission in any way. One public health official urged civilians to not think or speak of the flu as a way of preventing its spread. Although germ theory was known at the time, not enough was known about the influenza virus itself to enact any effective prevention. In fact, many scientists at the time believed that influenza was caused by a bacterium; viruses were not yet discovered.

It's pretty clear to see how global mobility and lack of control measures led to the rapid spread of this second lethal wave. But why was this so lethal, particularly to those populations who are usually most resilient to flu infections? Usually influenza mortality curves look like a U - highest mortality in the very young and the very old. The mortality curve for the 1918 flu was flipped - highest mortality rates appeared in people aged 20-34, at a case fatality rate of up to 10%. A couple things were going on to cause this phenomenon. One is that older populations had probably been exposed to a similar strain of flu in the past, which would have granted them partial immunity to this deadly strain. The other is that the immune systems of the young healthy people were so strong that they overreacted to the infection. Many deaths due to influenza were caused by an overactive immune system fighting against the virus rather than the virus itself. This is referred to as a cytokine storm. Finally, some historians suggest that heavy aspirin use, which was popular at the time, contributed to the deaths. Aspirin should not be used in treatment of flu, as it leads to complications. In any case, no effective treatment or cure for influenza existed (which remains true today to an extent), and as a result, 50-100 million people died worldwide.

Sources: The Great Influenza by John Barry; Flu by Gina Kolata; Pale Rider by Laura Spinney; The Fatal Strain by Alan Sipress.

TL; DR: The flu hit so hard because of massive troop movement, a unique immune system reaction, and lack of public health control measures.