Depends on what business you are referring too...If it is takeaway or dine in shops, I can understand why, if businesses are booming in a suburb, wouldn't you want a piece of the cake (profit). Another idea is, if the business is providing a particular, specific product or services, and if the company is a large corporation, they would want to set up business in a very hot market (suburb/s) knowing they can afford to lower their prices and have customers loyal to other businesses already set up, to shop at their place instead... This allows the new, big business to squash the little players as smaller; family owned businesses cannot afford to lower their prices to compete with the new shop.
A popular reason is simply because there is a substantial demand for that product or service in the area. Many firms offering the same service/product(competition) means there are many people who want said service and product. Most companies will offer the service or product in a unique way to build a niche market for themselves. Another reason is profitability. An example is Oil. Oil is a standardized product with very fierce competition. When an oil company increases the efficiency of their brand of oil by even a 10th of a percent(0.001) the payoff is in the hundreds of millions.