Why are fractal antennas able to receive so many more frequencies than a conventional one?
" fractal antennas are used in cell phones"

That is a bold statement. I would say "some cellphones use fractal antennas" and "Some fractal antennas are used in cellphones". Most antennas I have seen on cellphones are more traditional types like pifa or microstrip.

All antennas have a certain frequency band or maybe several bands in which they are most sensitive. It is a result of the design of the antenna.

All signals can be seen as a combination of sinewaves with each a certain frequency. Depending on the frequency characteristic of the antenna the antenna will pick-up that frequency better or worse than some other frequency. This is not a property that only fractal antennas have. All antennas behave this way. Some antennas are more broadband, some are more selective.