Because light, at the right frequencies, interacts weakly with matter.
This means that the surrounding temperature doesn't disturb the quantum state of light as much as it does when the quantum information is carried by matter (atoms, ions, electrons, superconductors, etc.).
For example, one can encode a qubit in the polarization state of a single photon, send the photon through air at room temperature, and find its state to be mostly unperturbed (of course, the longer the distance the worst the transmission fidelity will be).

A single low-frequency photon interacts weakly enough with matter that it can be sent very far before its quantum state is significantly perturbed.
Indeed, it is even possible (as in, it has been done) to exchange quantum information via photons with satellites, or set up intercontinental quantum communication networks.

Unfortunately, light also interacts *extremely* weakly (as in, it basically doesn't) with other light.
This means that two photons do not interact with each other, which makes it hard to implement basic components like two-qubit gates, when the qubits are carried by different photons.