This avoided the delays involved in having the head(s) seek to the correct track, and was also mechanically simpler (an important point at this early stage of development), but did require more components.
The very first drums were actuallly used for main memory on some very early computers; and also on slightly later, but low-end, ones (e.g. the IBM 650). Some drums were actually used to store registers, storing several registers in each track, and using multiple heads on that track to give faster access to the contents of the registers.
Drums fell out of use because they took more physical space than a disk with the equivalent surface area. A number of disk platters, given in total a significantly greater amount of storage, could be stacked in the volume occupied by a single drum.