I've done a bit of research into this, and can provide a bit of insight. Most of my answer is guided by Chacon et al.'s The Taking and Displaying of Human Body Parts as Trophies by Amerindians (2007) (particularly Robert Mensforth's chapter Human Trophy Taking in Eastern North America During the Archaic Period: The Relationship to Warfare and Social Complexity) (the "why" portion), and the writing of James Axtell - professor emeritus of history at the College of William and Mary - who addressed the genesis of scalping as a practice (the "where" portion), particularly in his article in American Heritage (volume 28, issue 3, 1977).

As a preface: I represent and work with quite a few tribes and many tribal members/allottees around the Western US, and can say - based on my personal experience - the vast majority prefer and use the term "Indian" themselves.

Why

Trophy taking in tribal North America has been associated with hunting in all kinds of native societies, and Mensforth wrote that the "planned acquisition, selective retention, and curation of animal body parts played an important role both in rights of passage to adulthood and in quests for personal esteem and prestige... Many warfare-related behaviors practiced by foragers are derived directly and logically from hunting behavior itself... warfare-related human body reduction often mimic those of hunting-related animal carcass processing..."

Thus, his premise is essentially that trophy taking from people was a natural extension of trophy taking from animals. This is because of the similarities between hunting party and war band dynamics in early tribal cultures, and the similarities in hunting party and war band planning and strategy.

It's important to note an interesting and unique factor when it comes to North American tribal hunting and war strategy; for thousands of years, the same group(s) of men (and sometimes women) spent virtually their entire adulthoods working very closely together actively tracking and ambushing big game and warring tribal bands. Despite some inherent differences, it can be reasonably said that hunting and warfare generally shared the same structural, social, foundational focus among those groups. As such, hunting and warfare rituals were often largely or entirely analogous.

Mensforth also answers your question in perhaps the most "literal" and logical way: the scalp was likely considered an easier alternative to a human head as a trophy, because of it being physiologically easier to remove from a body or wounded/captured person, easier to transport, and easier to preserve for subsequent display and adornment.

Where

It's believed one of the earliest European accounts of scalping was recorded by Jacques Cartier during his exploration of the St. Laurence in 1534, when he encountered the Stadacona band of the Iroquoian people. Their chief, Donnacona (who Cartier actually brought back to France) presented 5 scalps taken from members of a neighboring tribe that Donnacona had attacked (and allegedly massacred) the year before.

Around 30 years later, on Jean Ribault and René Laudonnière's 1564 French expedition to Florida, expedition member and artist Jacques le Moyne (famous for his early depictions of Native American culture and North American flora) wrote what Axtell claims to be the next (2nd known) European account of scalping; allegedly carried out by warriors from the Timucuan tribe, along the St. Johns River. Observing them, he wrote:

"[t]hey cut the skin of the head down to the bone from front to back and all the way around and pulled it off while the hair, more than a foot and a half long, was still attached to it... they dried the scalps until they looked like parchment..."

So, from what Axtell considers the first European-recorded instance of scalping from the New World in Canada (near modern day Quebec City) in 1534, to what he considers the next recorded European-recorded instance of scalping along the St. Johns River in Florida, around 29 years had passed. Straight away, from European accounts alone, there is not enough substantive information available from these two early accounts to make a sound geographical presumption as to where "scalping" began. Indeed, the locations of these two early European accounts suggests the practice was relatively widespread - at least throughout the Eastern part of the continent - by 16th century.

Archaeology also confirms, more objectively, the theory that it was indeed widespread. Axtell notes that the practice has been confirmed by archaeologists in pre-Columbian settlements along the Missouri and Mississippi Rivers, and in Ohio, writing: "The trophy skulls found in several Hopewellian burial mounds in Ohio, for example, frequently exhibit superficial cuts, apparently made by flint knives in the process of removing the flesh... In a number of prehistoric sites, circular lesions have been found on the skulls of victims who survived scalping long enough to allow the bone tissue to regenerate partially, leaving a terrible scar."

Thus, this confuses the geographical genesis of the practice even further.

Axtell conducted an interesting tribal language etymological review of the verb "scalp" in his 1977 article on this subject, as well:

"The older English word scalp did not acquire its distinctly American meaning until 1675 when King Philip’s War brought the object renewed prominence in New England. Until then, the best expressions were compounds such as 'hair-scalp' and 'head-skin,' phrases such as 'the skin and hair of the scalp of the head,' or the simple but ambiguous word 'head.' Likewise, the only meaning of the verb to scalp meant 'to carve, engrave, scrape, or scratch.' Consequently, English writers were forced to use 'skin,' 'flay,' or 'excoriate' until 1676 when the American meaning [scalp] became popular."

Interestingly, the tribes of the Northeastern part of the continent had relatively well-developed verbiage for the practice. The Huron, Iroquois, Mowhawk, Delaware, Oneida and several other tribes actually had particular nouns and verbs for a scalp, scalping, and even the victim of scalping / the "scalped."

Another instance important to the history of scalping as a practice - one Axtell did not write about (as most of the site was excavated in 1978, a year after his writing on the subject) - is the archaeological site of the Crow Creek Massacre, now a National Historic Landmark within the actual Crow Creek reservation in South Dakota. This massacre took place sometime between 1300-1360 AD. Archaeologists from the University of South Dakota found remains of 486 people (at least), most of whom showed signs of ritual mutilation - the most common being evidence of scalping. The September 1980 issue of the Smithsonian does an interesting, albeit brief write-up on it. Larry Zimmerman is the leading published author on the massacre.

So, based on the current understanding, the answer to "where the practice originated" portion is considered largely unsettled. One thing is clear, it is a pre-Columbian practice, and evidence of it can be found all over North America.

Edit: grammer!