Lifespan of 50 - 75 years in the ground is probably the reason. I have tried a few small one's potted that just plain up and died after a few years... just my thoughts on it but there could be other factors as well - they are rather large leafed.
Grimmy
Not trying to pick a fight, but when one quotes ''lifespan'' for a tree you need to consider the source of the data. Trees have no ''lifespan'' in that their cell lines are indeterminate, and can in theory keep on growing and reproducing for centuries. This is true even of the trees listed as having very short life spans. If you dig into it, the origins of life span data comes from two possible sources. The lumber industry will publish a lifespan as the age at which a multiple acre or hectare planting of trees for lumber will have matured to the point where a significant percentage of the trees in the stand will have begun to have hollow trunks. I believe I remember the magic number being 20%, but I could be wrong on the exact number. This is then used as the latest or longest interval you want a timber planting to mature before the next clear cut harvest. This is what the timber industry means by life span, harvest a stand at an age younger than the lifespan to maximize board footage or $$$ yield.
Second source is the landscape industry. Here they list life span as the time a tree will look attractive with relatively low maintenance effort in a well groomed modern landscape. At the end of the lifespan the tree will either be too large for its site, too gnarly to be viewed as attractive or disease susceptible enough that likely to have scars, hollows, broken branches or other features that would be considered ugly in a modern landscape. It does not mean the tree is dead. It means the tree gets replaced by smaller younger material allowing the landscapers to make money removing the old and money again by planting the new.
So regardless the original source of the date, neither group uses lifespan data to mean the tree is actually dead or actually dies, they use it to mean the tree is no longer useful for their industries' purposes, to make money.
It is possible to keep a redbud alive for hundreds of years. They do not go senile and die the way humans or dogs would.
Once initially published in industry related trade literature, these lifespans are often quoted out of context. Tracing back to the original research showing the criteria used to determine ''lifespan'' can be multiple levels of one article quoting an older article, quoting an older article. It can be difficult to find the source, but when one does, you will see, the criteria used to determine lifespan was something other than what we normally think about when considering life span of animals. Plants, trees in particular, are quite different than animal in this regard.