The era prior to Jumpships only produced 2 interstellar colonies, There were two primary reasons for this. The first of these was technical: starships of sufficient speed, reliability and capacity could not be built easily and sub light travel really was just too slow. Nonetheless, it was not impossible as the establishment of the first two colonies show.
The main reason was economic, the megacorps and interplanetary powers were far too busy exploiting the solar system and for projects of such risk and timescales. Significant interstellar  colonisation was not achieved until the development of the Jumpship in 2400. Even though the Solar League  was experiencing a major economic and social boom, but it was clear that resources were running out, and habitat space was not growing fast enough for the booming population.
So it was fortuitous timing when the SS Prometheus was launched in 2380 but no so fortuitous when she was lost making a warp transition a year later during trials. It took nearly 50 years for humanity to make a safe Jump Engine, and it was only because of the development od 'Super AI', the archai.  
The combination of Jumpship technology and the archai resulted in a burst of colonization in the early years of the Diaspora, as various groups attempted to grab entire star systems
Colonization collapsed after the Great Exodus in 3102 and for the ensuing Dark Ages, and colonisation did not resume until the Second Solar League era.
The Solar League strict limits on new technologies, especially archai, hampered the solution to this problem, and was a brake on market growth in general. Hence, aside from the simple press of population many organisations saw interstellar colonisation as a way not only to gaining access to resources but to exploit the potential of technology more freely. Other another strong reasons for colonisation were safety (many feared the return of a new Dark Age) or a desire to escape the fierce competition of the solar system.

The new colonisation ventures had profound political effects over the long term. One was the gradual weakening of the Solar League as a social and political force, as more and more activity went on outside Solsys. 
The AI-supported hypereconomy was stable and to a large extent predictable, and the colony projects would lead to multi-decade planning horizons. While a long range plan of the Information or first Interplanetary ages could have been five years, now fifty or a hundred years became a rational planning timescale. By putting more and more decision making and management power into the hands of the immortal and reliable AIs, corporations could manage even multi-century operations without any risk of losing control. Also, company policy could not be allowed to change too quickly, or the projects would go haywire due to communications lags. This was the first step along the road which eventually led to the Age of Empires and the transformation of the megacorps into hereditary houses.

The selection of colonisation targets was based on several factors, but distance, the presence of suitable asteroid/kuiper belts and the presence of habitable/terraformable planets were, in that order, the most important.  As for habitable planets, terraforming had to compete with tweak adaptation in the initial era. Terraforming was a good long-term investment, but adapting tweaks or exploiting the bewildering array of clades of the solar system proved to give a much faster benefit

The introduction of jumpships changed the pattern, turning up the interest for terraformable planets around sun-like stars. Terraforming rose to prominence, but still required ever longer time scales of planning. Low-tech terraforming can take many centuries, and the hi-nanotech approach could work within a few decades, the main limitationn being cost.  
Terraforming investments in the late First Federation proved to be one of the most profitable forms of long-term investments ever. By sending a terraforming team to a planet a nearly worthless chunk of rock could be turned into prime colonisation and economic growth territory within a century or two (by now an ordinary investment horizon for corporations and long-lived individuals). The value of the planet would in turn increase nearly exponentially - apparently forever. Terraforming and colonisation funds became funds of choice, and accelerated the colonisation process as they in turn invested in more colonisation ventures.

Over time colonisation began to develop a stable dynamic. A vast wave of exploration and surveying probes expanded at near lightspeed outwards, relaying their information back to their owners through encrypted message packages. Interesting systems were visited by linelayer ships and attached to the wormhole nexus. Many systems were selected as wormhole outposts with receiver arrays just to keep near to the information front (a delay in receiving a tip about a good system could mean that a competitor got there first). The linked systems were then developed as before, but by now the technologies were standardised and routine. One result was the tree-like spread of empires, with wormhole networks radiating away from Weylforge systems into the frontier. Large volumes were left untouched as too far from any suitable linelayer or too uninteresting; in these volumes other empires and independent worlds sometimes appeared.