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.
Extreme lack of supplies could exist in this planet that causes this species to spend nearly all of its time scavenging compared to engineering