One reason was that in the late 18th century, Britain was trying to define who was "British." And this, led in turn to the question of what a "colony" was and "how much" representation one should have. There were two kinds of colonies.
The first kind was fundamentally empty land (America, Canada, later Australia), settled mostly by people of English descent. These colonies could be considered "transplanted" parts of England. Some, but by no means all, people on the British Isles would consider these people British.
The second kind of colony was territories like India, and late Africa, that were conquests, rather than settlements where "Englishmen" established their rule over non English peoples. Few people anywhere would consider the "natives" of these lands British.
Culturally, it was easier to give representation to the first group of people, who were (mostly) of English descent anyway. But the logistics of the time made this difficult; there was no air travel (only sea), and no telephones or radios to connect e.g. England and America. At best, the colonies could elect represenatives who would be seated in London. With Canada and Australia, England learned from the American example, and also benefited from the better transportation and communication of the 19th century. Perhaps in those cases, it was a matter of when, rather than whether they would gain representation.
With the second group of colonies, the question was one of "who is an Englishman and who should be represented?" England was a very class conscious society at the time, and controlling for education and economic status, there were large social gaps between being a Londoner or a resident of a northern city like Liverpool or York. Within the United Kingdom, the difference between an "Englishman" and a Scots, Welshman, or "orange" Irish was significant. After that, there were major (perceived or real) differences between people in the British Isles and those "offshore" as discussed in a previous paragraph. Finally people like "Indians" never got anything like full representation; if they had, they would have "swamped" the British empire on sheer numbers.