Question: How could a really intelligent species be kept from developing? For clarification, this species is the only creature of intelligent thought on the planet and they have a population of about 50 million across the planet about the size of the moon. However, they can't develop to the level that Britain reached in the industrial revolution.

Answer: 
Two big factors:

1. Limited resources. If everything is expended to support themselves, they will not have the faculties to invent new things. They may even spend their idle time sitting about so as to not waste more calories when they don't have them.
2. Local maxima. If they have achieved peak effectiveness such that incremental changes will not make the situation better, any attempts to innovate will be a waste of resources, because great leaps are rare. Also this discourages innovation because it's a pure waste.
No need to plan ahead
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In our history, complex civilizations developed where there was a need to plan ahead. In temperate regions, like Europe and China, people had to stockpile enough food and firewood to survive the winter. In the Middle East, they needed irrigation because of the arid climate. And in all the above cases, growing food was seasonal: there was one big harvest in a year, and that harvest had to last for an entire year. This required a complex organization of society, and that society needed to organize its protection, because if your neighbors had one bad harvest, they would either starve to death or had to try to raid you.

Compare this to tropical regions where you can pick food from the trees all year round, and although life as a hunter-gatherer is not easy, one does not need to plan ahead for an entire year. Humans there remained on a tribal level just as they lived tens of thousands of years ago, until fairly recently when they were contacted by more advanced civilizations.