Why does ionizing radiation cause only DNA double strand breaks?
Ionizing radiation does cause single-strand breaks and other kinds of DNA lesions that are not double-strand breaks.  However, double-strand breaks are the most difficult to repair and are thus the most likely DNA damage type to result in mutation and / or cell death.  See Fig 1. from DNA damage response signaling pathways and targets for radiotherapy sensitization in cancer:


DNA damage induced by ionizing radiation. The major types of DNA damage induced by IR include base and sugar damage, single-strand breaks, double-strand breaks, clustered DNA damage, and covalent intrastrand or interstrand crosslinking