Why Transferring binary files requires a mode that handles full 8-bit data and guarantees error-free delivery. PACTOR does exactly that: it uses ARQ (automatic repeat request) with acknowledgements and data compression, so any block received with errors is resent until it is correct, making it suitable for file and email transfer over HF. The other listed modes are keyboard-to-keyboard chat modes built around limited character sets.
Watch out AMTOR is the closest relative and also uses ARQ, but it is based on 5-bit Baudot-style code and can only pass a restricted character set, not arbitrary binary data; PSK31 uses varicode text and RTTY uses 5-bit Baudot, both without error correction.
PACTOR = packet + AMTOR: it inherited packet's 8-bit binary data handling, so it is the file-transfer mode.