It grew by leaps and bounds because trucks could easily undercut the artificially high, fixed price maintained by the ICC for the railroads. Trucking was a relatively low capital operation, therefore fairly easy for someone to start a trucking business and thus highly competitive. The railroads lobbied for government regulation of trucking and trucking was brought under ICC regulation in 1935. There were 89,000 applicants for trucking certificates at the onset of regulation, the ICC approved only 27,000. Through the years until deregulation the number of trucking companies decreased while the tonnage carried grew tremendously.
Most cities regulate taxis, usually limiting the number of licenses issued and setting rates.
It, like the ICC above, limits entry into the market, thus reducing competition. It also hurts those who wish to start a taxi business, because in all likelihood they won't get a license. The existing cab companies are protected from competition. Once again big corporations use big government to protect themselves from competition. They pay their players millions of dollars every year to play.