Yes, topography. The cliches about there being no reason for Dallas to even exist refer to the fact that it does not sit on a major body of water -- the Atlantic Ocean or Mississippi River for example. True enough, but the three forks of what came to be called the Trinity River were "major" at times if you had to cross them on foot or horseback or in a wagon pulled by a team of oxen. Often no more than a large creek in dry seasons, the river can be very imposing during the rainy season.
Settlers first came to this area while traveling the Shawnee Trail.
In 1838 the Congress of the Republic of Texas appropriated money for the construction of a north – south road, there by opening northern Texas to trade. The Shawnee Trail from Austin to the Red River was followed for this route. This road, ran through the heart of what later became the City of Frisco.
A military post near the Red River was named for Captain William C. Preston, a veteran of the Texas Revolution. The Shawnee Trail, which would ultimately become Preston Trail, then Preston Road, was used by wagon trains moving south bringing immigrants to Texas and by cattle drives going north from Austin. The town, Lebanon, then a thriving a cattle town and now a part of Frisco served as an assembly point for the cattle drives. South of this area in 1841, John Neely Bryan began the settlement of Dallas.
In 1838, the Republic of Texas had passed legislation and authorized funds to survey a road from the Austin area to the Red River. Colonel W.G. Cooke began the survey in September, 1840, and reached the Red River near Coffee's Trading Post in late October after crossing the Trinity River near where downtown Dallas sits today. At the time, Coffee's was the most important post on the Red River west of the Louisiana Territory. The budding and ambitious Texas Republic recognized the importance of Coffee's Trading Post and decided that the road north from Austin should hit the Red River near Coffee's. Nearby, an important little town was springing to life -- the town of Preston. The northern leg of the road, from the "three forks of the Trinity River" to the trading post near the town of Preston quickly became known as the Preston trail...or Preston Road! Ever heard of it?? If you have never been in Dallas, Preston Road is one of the city's premier addresses. It runs north from the Turtle Creek/Oak Lawn area through Highland Park, North Dallas, Plano, and beyond. Now, get a map of Texas, and use a straight edge to connect Austin and Denison. The old Preston townsite now lies at the bottom of Lake Texoma, about 12 miles west of Denison. So, that straight line from Austin to Denison crosses the Trinity River at the BEST place to cross the Trinity River..at Dallas! Obviously, had Colonel Cooke's destination been a hundred miles east or west on the Red River from Coffee's Trading Post near Preston, the planned road would not have crossed the Trinity River at the site that is now Dallas.