It is most likely because of the animals traits, for instance, a fox would do better in a city than a deer because foxes are not fussy about their diets while deers are. Also the animals may already be more physically adapted e.g. a fox has claws which can be used to access hard to reach food where as a deer has hooves which would most likely not help it reach food.

these animals are known as synanthropic, literally 'together with man' although widely regarded as pests, i believe we should celebrate these animals for their ability to adapt to the environmental pressure applied by humanity. the reasons for certain animals ability to coexist so are varied.

urban foxes owe their success to their adaptabiltity. they have adapted their predominantly carnivorous diet to a scavenging one. they have also been able to take advantage of the land in our cities that we don't use, wasteland, raliway line embankments etc as places to build dens and raise their young in realtive peace. their nocturnality has also worked in their favour allowing them to share our environment without coming into contact with us too often.

the prevelance of pigeons in our cities is mainly due to the availability of roosting and nesting sites similar to what ancestral pigeons would have used (tall, sheer sided buildings with precarious ledges being very similar to cliff faces). pigeons have also benefited from the exclusion of their natural predators from cities (although raptors such as peregrine falcons etc have been increasingly attracted to take advantage of large numbers of prey) finally due to the wastefulness of humans, there is always plenty of food available for pigeons (discarded chips, kebabs and the weekends bounty of leftover vomit)

seagulls are a more recent arrival in our cities, their exodus was triggered by a change in waste management practices in the 20's. before this most waste was incinerated, it 's only when landfill became standard practice that gulls began to move in. since then they have adapted incredibly well to their new niche, becoming more aggresive and according to some eyewitness accounts, even beginning to prey on pigeons (http://news.bbc.co.uk/1/hi/uk/1459804.stm)

it is not only animals that live within our cities that are described as synanthropic, rats and mice have been with us going back to prehistory. parasites that coexist solely with humans (headlice, pubic lice etc) are also known as synanthropic.