A Brownfield Project is a project that already exists and therefore comes with a potentially host of constraints, dependencies and intricacies.
Aside from the technical, there is also often a level of human entanglement, that consists of time-honored conventions, traditions and doctrines, which people grew fond of or got used to and - since change is always difficult - find challenging to modify. Even when suffering from the status quo, people prefer to keep it more often than not.
