It is is not just about being expensive, but potentially fatal if things go wrong; you are editing immune cells that perform very dangerous tasks that require being able to tell apart "normal" body cells from "diseased" cancer cells (in this case cancer cells with mutated genes).

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If the now-reprogrammed immune cells start attacking healthy cells, or if the process of gene editing itself goes wrong and during insertion of the genes into the host there are "unintended rearrangements" (basically oopsies) it could lead to a plethora of things, including but not limited to; chromosome rearrangements and/or chromosomal instability if too much of the DNA is damaged directly or indirectly during "repairs", leukemias, etc.

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Some of these patients were receiving immuno-depressing therapies for more than a year! You have to balance other medications that goes with it, and let me tell you something about research with the sickly; it is expensive mentally, physically and economically to not only patients, families, practitioners and and all other supporting staff, but you also have to take into account that rarely do these patients present with only that cancer, chances are they are dealing with other concurrent diseases like diabetes or other metabolic syndromes, as well as socioeconomic barriers preventing them from getting proper care.

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Many things can go wrong, you can throw millions, if not billions of $$$ at it and it is still does not solve the fundamental problem because capital is not the choke point.