From Wikipedia - Floppy_disk:
to format or write to this high-coercivity media, the high-density drive switched its heads into a mode using a stronger magnetic field. When these stronger fields were written onto a double-density disk (having lower coercivity media), the strongly magnetized oxide particles would begin to affect the magnetic charge of adjacent particles. The net effect is that the disk would begin to erase itself.
This is a similar question to Was there a special drive, that saved much more than 1.44MB on an HD floppy disk?, but more dedicated:
How long can a MF-2HD disk hold 2.88MB of data, before it begins to erase itself?
And can any firmware hack allow enforcing 2.88MB onto 1.44MB?
The 32MB squishing technology from SuperDrives is different and requires hardware dedication.