why uranium does not have any stable isotope?
Well, this is the same as asking why every isotope of U has at least one open decay channel. This has to do with the binding energy of these nuclei and those that are connected via decay channels. Basically a restatement of your question. You could look at nuclear shell model and such. Hope this helps.
Did you notice that after Pb there are no stable isotopes at all ?
yes.. the qus goes for all of those.
What do all nuclei above Pb have more of than those before?
To greatly oversimplicate things, it boils down to how the repulsive force from protons competes with the attractive strong nuclear force, with a dash of quantum mechanical rules sprinkled in. Or, rather, dumped in by the gallon since this is absolutely a quantum scale system that can't be analyzed classically except to approximate things to various degrees.
As Paul Colby said, you can look at various models, such as the nuclear shell model, to find out more info.
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