There are 2 reasons why the redefinition has been postponed by a couple of decades.

One reason is that the best optical clocks are very complex and it can be difficult to make them operate continuously for a long time. Until more reliable clocks will be made, the only solution for maintaining a continuous time scale would be to keep in each laboratory a relatively large number of optical clocks in operation, so that every time when one or more are halted for maintenance there would remain enough clocks still running. This is feasible, but expensive.

Besides waiting for more reliable or cheaper optical clocks, there is one more fundamental reason why the redefinition has been postponed.

When the cesium clock was chosen for the SI definition, there was no alternative with comparable precision, so the choice was obvious. Now there are about a dozen of different kinds of optical clocks with similar performances and it is not clear which will become eventually the best.

Therefore there exists a proposal with which I agree, that the future SI definition should no longer be based on a single frequency standard, but the unit of time should be derived from a frequency that would be the weighted geometric mean of the frequencies of many kinds of optical clocks, perhaps 5 or more of them.

Such a definition would also give the possibility that depending on the evolution of clock technology some standard frequencies could be added to or deleted from the set of frequencies used to compute the mean, or their weights could be altered.

The fact that thorium 229 can be used to make a nuclear clock has been discovered many years ago.

Nonetheless, this has become possible only today, after many years of researches. There is no doubt that future thorium 229 clocks will be better by several orders of magnitude than these first 2 prototypes, so it is likely that the corresponding clock frequency will be added to the set of frequencies on which the future SI definition will be based.