It's easy to drill a straight through chamber, and most early cartridges were rimmed. So a chamber was simply a hole the size of the cartridge. It's easy to cast lead into any reasonable shape, so by making the bullets fit both the inside of the case (where the heeled portion is) and the chamber (the portion with the band in the picture), which was the same diameter as the outside of the case, the manufacture of the chamber was much simplified.
When velocities jumped with the advent of smokeless powder, jacketed bullets started becoming more common and heeled bullets began to fall out of use for several reasons. One reason is that it's a pain to make a jacketed bullet with a heel, so the bullet was made to fit inside the case, and the chamber was tapered down in front of the case mouth to fit the bullet diameter. In addition, in rimless cartridges, the chamber had to provide a means to headspace the cartridge, so more complicated chamber shapes became commonplace. So it was no longer an issue to make a chamber that wasn't a straight through hole, and the higher velocities and other issues made heeled bullets less attractive.
The change away from heeled bullets also helps explain the apparent disconnect between some caliber designations and the actual bullet diameters commonly associated with them. For example, the .38 special uses a .357" diameter bullet. The reason is that originally, .38 revolvers used heeled bullets, with a nominal bullet diameter identical to the OUTSIDE case diameter. Later, when heeled bullets fell out of common use, .38 revolvers transitioned to non-heeled bullets with a nominal bullet diameter identical to the INSIDE case diameter which was about .357". Same deal with .44 revolvers which typically use .429" bullets.