Slot allocation rules are laid down in an EU Regulation of 1993, and subsequently amended twice (once to stop airlines being unduly penalised by the cancellations they made during the SARS outbreak).
The introductory paragraph on the EU site states the following:
One of the main difficulties of the current system of slot allocation has been to find the right balance between the interests of incumbent air carriers and new entrants at congested airports so as to take due account of the fact that incumbent air carriers have already built up their position at an airport and have an interest to expand it further, while new entrants or air carriers with relatively small operations need to be able to expand their services and establish a competitive network.