These dual layer rewritable DVDs are DOA technology just as Sony's DDCD and Sanyo's HD-BURN
Some factors that caused it never been released:
Manufacture cost is rather high. Dual layer media is difficult to produce. Notice that DVD+-R DLs are much more expensive than SL. A spindle of DVD+R DL are twice to third as much as a spindle of BD. The cost of rewritable DVD±RW DLs would be even higher.
Compatibility issues. DVD±RW DL will be incompatible with current hardware. This is due to optical signal levels lower than prerecorded media that will require hardware and firmware to be adjusted to accommodate the differences. Analogous to CD-RW’s introduction in 1996, neither existing DVD recorders nor DVD players will be able to write or read rewritable DLs. History repeatedly teaches that such incompatibilities bring only customer confusion.