If there was a single word which explains which explains why the trading pits have closed everywhere but the US, it's "disintermediation' - the process of removing middlemen from transactions." Various forms of disintermediation continue to exist in the trading world and recent software developments are speeding the destruction of niches once thought to be permanent.
These themes were outlined quite well in the book Capital Market Revolution by Patrick Young and Thomas Theys which was published in 1999 during an age that trading pits were in the process of shutting in Europe and the decline of open outcry began in the US. Author Young is a trader/journalist and author Theys is the founder of trading software firm Patsystems so they were very well qualified to describe the changes which happened and in many respects continue to happen across market structures.
The book is as much a retrospective on why the changes happened as much as predictions for the future of which some were realized but others remain unfounded. At the time of publishing in 1999 it was a bold book which largely forecast the trajectory of the industry and remains fascinating for the lessons.
Viewed as how the industry has changed so much in the past decade, the introspective questions Capital Market Revolution posed were not reflected upon by many participants. Although I won't compare it quite on a level as my favorite business book of all time, Only The Paranoid Survive by Andy Grove, there are many similarities between the two.