South East Asian region comprising of current Indian and Pakistani territory has been considered to be the nurturing grounds for a language developed some 900 years ago, and evolved through different phases of history to develop into what we know today by the name of “Urdu”.
It was mostly taken as native language of Christians abode in Karachi (Pakistan), Rajhistan, and Uttar Pardesh (India) during the 20th century, and is still used by considerable majority in Indian states, while being exalted to the status of national language in Pakistan.
During those time, majority of people living in those states adopted Roman Urdu as a more convenient medium of written communication as compared to a bit tricky and demanding original Arabic-Perso script used by conventional Urdu.
It was a common medium of written communication of amongst the Christians residents of those areas till 1960s. The Bible Society of India even published (still does, but to lower extent) Roman Urdu Bibles, which were quite in demand during that time; but the eventual promotion of Hindi in Indian states and traditional Arabic-Perso script in Pakistan diminished the advent of Roman Urdu.
The conditions remained like this till Bollywood (popular name for Indian film industry) started using Roman Urdu as one of the main scripts for its movie titles (along with Hindi). Since the influence of Bollywood film industry has increased quite significantly since the last quarter of the preceding century, it has helped Roman Urdu resurface all over again.
In addition to that, people have started using Roman Urdu as a convenient mode of written communication via SMS (short messaging service) through mobile phones. It has also contributed in re emergence of Roman Urdu with greater significance.