The MN32xx series of delay chips was essentially developed to address a circumstance arising from battery use, which would have been more typical at that time. With the rapid growth in both digital pedals (which couldn't survive more than a few hours on a battery), and mini-pedals that have no room for a battery, pedalboards with external power-supplies are now the norm, making the need for the 32xx series a non-issue. ***
Panasonic/Matsushita abandoned production in the mid-'90s. Until just a few years ago, when Xvive started making MN3005 and 3007 chips again, pretty much all we had available were 32xx chips, made by Beiling and later on, Coolaudio (a Behringer spinoff/subsidiary/supplier). I don't know why it took so long for 30xx chips to return. Coulda been something legal, coulda been a question of fabrication difficulties, coulda been a matter of perceived market/demand. I have no idea. EHX uses a quartet of them for their various analog delay pedals, so that provides a pretty steady demand for production.
Are the 30xx chips "better"? Some folks say so. They do have a little more dynamic range and better S/N ratio, but the threat to noisy delay pedals comes much less from the delay chips, and more from the clock used. The common misunderstanding is that the ability to run off 15V, rather than 5V magically translates into much greater headroom. But the reality is that the actual headroom in bucket-brigade chips is rather limited, and restricted to a mere fraction of the supply voltage. It's not as if the signal swings nearly 15V instead of just under 5V. There IS an improvement in headroom, though, just not nearly as much as presumed.
*** Why does the supply voltage matter? All bucket-brigade chips need to be "biased", such that the audio signal rides in on a DC voltage that the chip needs to simply pass signal. The bias is derived by dividing down the supply voltage. This involves a trimpot that you will find n pretty well every analog time-based pedal, whether delay, chorus, flanger, doubler, or vibrato. However, deriving 14/15ths of 9.6volts (a fresh alkaline battery) is not the same as 14/15 of a battery that has dropped to 7.8V. Manufacturers could not expect users to constantly adjust bias as the battery aged, so Panasonic developed the alternate 32xx (and later 33xx) series of chips. They could run off 9V, but could also run well off 5V. A 9V battery can be dropped down to a regulated stable 5V until the battery itself declines to 7V, which provides sufficient lifespan. The bias voltage derived from that stable 5V supply remains stable and needs no further adjustment, such that the pedal behaves consistently. But again, all of that is a practical response to running a pedal off a battery. If you're running off an external regulated supply, it adds nothing to the reliabile performance of the pedal.