That happened because the code changes from ext2 to ext3, and later from ext3 to ext4, were very invasive, and filesystems is one place where stability and reli

That happened because the code changes from ext2 to ext3, and later from ext3 to ext4, were very invasive, and filesystems is one place where stability and reliability is very important. So instead of risking introducing data-loss bugs, they forked the code. It was always the plan that the older code would be removed some time later, once the newer code had proven its stability for a while. The on-disk format for these three is the same; the difference is that the later ones understand more of the "incompatible" additions to the on-disk format (IIRC, the main ones being the journal for ext3, and extents for ext4, but there are others). You can convert an "ext2" filesystem into an "ext4" one by just enabling the relevant filesystem format flags on the superblock, and making a few necessary adjustments (for instance, creating a journal when enabling the "has journal" flag).