It's all about stability. If you try and jam too many protons and neutrons together, they won't stay together for long. If the nucleus disintegrates before there's time for electrons to form and act like a chemical element, it's not really in the realm of chemistry any more.
"IUPAC defines an element to exist if its lifetime is longer than 10^−14 second, which is the time it takes for the atom to form an electron cloud.[7]"
"IUPAC defines an element to exist if its lifetime is longer than 10^−14 second, which is the time it takes for the atom to form an electron cloud.[7]"
https://en.m.wikipedia.org/wiki/Island_of_stability
https://en.m.wikipedia.org/wiki/Superheavy_element
(Edit: this was intended in response to ssijak's question about the theoretical limits)