As an agency worker, even if you're on a long-term placement with a company, there's no obligation on that company to offer you a job directly employed by them.
Whilst it might make better HR sense for them to offer you a permanent job, many employers would actively choose temps over employees. This is for a number of reasons, such as trying to avoid some employer responsibilities for employment rights - or trying to avoid paying your agency a "temp to perm" fee (compensation for losing you to the company).
Also, as of 11 October 2011 agency workers will have the right to the same basic terms and conditions as a directly employed worker (after they have been in the job for 12 continuous weeks).
