What do you mean limited funding? Seems like there's a TON of funding for cancer research right now, given the moonshot campaign (extra $1.8B over 7 years).
It's hard to study something like this, because you have to find a large enough population for the statistics to work out, but also keep the study going long enough without massive attrition (would have to do intent-to-treat) to have results worth publishing.
There is probably limited funding for this because even if it is true, it will not result in a large stream of revenue for anyone. In fact, if it works it might reduce some health industry organizations' revenues.
It's hard to study something like this, because you have to find a large enough population for the statistics to work out, but also keep the study going long enough without massive attrition (would have to do intent-to-treat) to have results worth publishing.