Basically just technology improving over time. Just looking at wikipedia, 5.25" was introduced in 1976 not clear at what capacity but it fairly early on settled

Basically just technology improving over time. Just looking at wikipedia, 5.25" was introduced in 1976 not clear at what capacity but it fairly early on settled into 360KB. The 3.5" was introduced in 1982 at 720KB with 1.44MB broadly used by late 1980's. The Zip drive was introduced in late 1994 with 100MB capacity and was later expanded up to 750MB. But by that time CD-ROM technology with its cheaper media and less proprietary nature was very much replacing the magnetic based portable storage media.