Also, maybe, the fact that what cancer cells have in common, they tend to also have in common with cells that aren't cancer, and therefore make poor targets for therapies.
That's kinda a defining characteristic of cancer. It's your own cells, so your immune system doesn't recognize it as foreign and take care of it. Just about anything that targets cancer targets the non-cancerous cells it started in. Most chemotherapy boils down to "kill it all and hope the cancer dies before the non-cancer, and leaves enough non-cancer behind to keep you alive". There have been a lot of advances in targeted cancer treatments, but even most of them just selectively target the type of cell (healthy or not) that became cancerous, and do less collateral damage to "innocent" cells. It's incredibly hard to target an individual cancer, and so far impossible to target cancers broadly.
I'm no expert, but I know cancer cells promote adding blood pathways towards them, so it might be that they have an "uneven share" of the nutrients transported to them, and thus die out later than the healthy tissue.
I don't know anything about cells going dormant as a result of fasting. That sounds questionable to me. There's been some research on fasting and cancer, but there haven't been a lot of studies, and results are inconclusive. Some of the ideas sound sensible. Cancer cells are incredibly greedy and slurp up glucose as they constantly divide. The thinking is that you can starve them and slow progression, in conjunction with chemotherapy and other treatments. It's not widely supported or practiced and more study is warranted. It's hard to find real information on it, because the idea that you can miraculously cure cancer and other diseases by changes in diet is embraced by millions of anti-science crackpots, so there's a ton of disinformation and speculation out there.