I think it is partly a result of the extreme difficulties of typesetting in the nasta’liq style. Until quite recently, almost all the newspapers in the country were handlettered. So there is a large number of professional signpainters and letterers, working with both Urdu and English, and a public understanding that if you want a sign painted you go and hire one of these people. This contributes to a culture in which there is an expectation of quality in signpainting, even in protest banners, that is not undermined or homogenised by cheaper and faster mechanical lettering options. You see the same sort of quality, although much more restrained in style, in late 19th and early 20th century suffrage protest signs in Britain.