Twitter advertising has an interesting history which has been heavily influenced by texting. However, texting has never been a huge ad platform so Twitter has yet to attain ad powerhouse status despite its rapid growth and popularity. One of the reasons for this is that too many ads will cause users to switch to competing services, but another reason is the vision of the Twitter founders to create an entirely new service which was free from traditional advertising formats like banner ads.
However, the Twitter founders were also pragmatic in realizing that they had to earn income for their service in some other way than charging access fees, so advertising is the natural answer. They took a slow approach in opening the doors to advertisers as the service grew, adding promoted tweets and promoted trends along the way.