TL;DR: Because today, people are regularly living long enough to get it. It's often a good sign to have higher cancer rates -- societies with higher cancer rates are richer, happier, and live longer than those with lower cancer rates.
I mean, cancer is bad. But it's a good sign for society if lots of people die of cancer -- they tend to hit the elderly. Historically, about 80 percent of people who die of cancer are over 50, and that's fairly constant. (Interestingly, the share of cancer deaths that are in people over 70 have been rising, from about 36 percent to 49 percent - that's as a portion of the total.)[1]
The ranking for prevalence of cancer is higher income countries, upper-middle-income countries, lower-middle-income countries, and finally with the lowest rates low-income countries. Since the world is getting richer, it stands to reason that it's likely there will be higher cancer rates (of course, it's not guaranteed; it's possible being European makes you more likely to get cancer, which would explain the higher rankings of high-income countries, which are often European, but not lead to higher numbers elsewhere).[2]
This is borne out individually. Countries with the lowest rates of cancer tend not to be great places to live. Our World In Data has three countries[2] tied for the lowest cancer rates (0.1 percent), Niger, Chad and Benin, which have life expectancies of 62, 53, and 60[3][4] and had happiness scores (self-reported life satisfaction) of 4.56, 4.47, and 4.38 (out of 10; for reference, the world average is 5.27, with numbers for individual countries ranging from 7.74 to 1.72) respectively.[5] The full list of countries with a prevalence of cancer below 0.5 percent is as follows:
Niger, Chad, Benin, Gambia, Somalia, Sierra Leone, Mali, Yemen, Democratic Republic of the Congo, Central African Republic, Cote d'Ivoire, Burkina Faso, Burundi, Liberia, Angola, Guinea, Cameroon Guinea-Bissau, Nigeria, East Timor, Tajikistan, Mozambique, Senegal, Togo, Papua New Guinea, Afghanistan, Ethiopia, Madagascar, Oman, Sudan, Nepal, Kenya, Mauritania, Maldives, Bhutan, South Sudan, Ghana, Vanuatu, Equatorial Guinea, Bangladesh, Tanzania, Solomon Islands, Sao Tome and Principe, Eritrea, Malawi, Rwanda, Laos, India, Uganda, Pakistan, Uzbekistan, Djibouti, Congo, Botswana, Zambia, Kiribati, Lesotho, Algeria, Gabon, Mongolia, Eswatini, Morocco, Comoros, Honduras, Haiti, Samoa, Zimbabwe, Guatemala, Turkmenistan, Myanmar, Kyrgyzstan, Marshall Islands, Namibia, Philippines, Egypt, Cambodia, Indonesia[2]
The happiest of those is Guatemala, at 6.29, and a prevalence of 0.4 percent. But most of them are much less happy. To take a random example (I used random.org to randomize the list and chose the top one and the top six, excluding Sao Tome which had no happiness data), Uganda is at 4.37, and the average of Uganda, Comoros, Turkmenistan, Yemen, and Bhutan is 4.41, nearly a point below the world average.
Life expectancy is no better; again doing Uganda, Comoros, Turkmenistan, Yemen, and Bhutan, Uganda had a life expectancy of 62.7 years, and the average of them is 66.2, almost 5 years below the world average of 71 (range of typical life expectancy is from 85.9 to 52.5).
Okay, so life expectancy and happiness are both lower for countries with lower rates of cancer, seemingly. What about those with higher rates?
Well, those with cancer rates up to or including 3 percent are as follows: Monaco, Bermuda, Italy, France, Netherlands, Finland, Germany, United States, Australia, New Zealand, Japan, Denmark, Portugal, Greece, Croatia, Spain, Estonia, Canada, Norway, Andorra, United Kingdom, Slovenia, Belgium, Iceland, Switzerland. Monaco comes in at the top, with 5.9 percent with some type of cancer.
Monaco has a life expectancy of 85.9 years, the highest. In the world. There was no happiness data available for it or the runner-up Bermuda, but Italy ranked itself at 6.32.
Of the top five countries in life expectancy that show up on the cancer list at all (Monaco, not Hong Kong or Macao, Japan, Australia, Switzerland, Malta), all of them but Malta show up on the list, and Malta just misses the cutoff (2.9 percent). In happiness (Finland, Denmark, Iceland, Sweden, Israel), Sweden and Israel aren't ranked. Sweden just misses the cutoff at 2.9 and Israel is at 1.6 percent but has a notably much younger population than any of the others.
Again using random.org and taking the top one and then the follow-up four, Monaco came in at number one (data already covered and missing happiness, so excluded), followed by the US, Switzerland, Iceland, Australia, and Italy. The US has a life expectancy of 77.2 years. The average for the five is 82.26, so if it were a country it would be number 21 globally and over TEN YEARS above the world average of 71, above even the Oceanian average (the highest) of 79.4; for happiness, the US is number 18 worldwide excluding ties, ranking itself at 6.72; the average is 6.938, meaning if it were a country it would come in number fourteen including ties and almost two points above the world average of 5.08.
[1] https://ourworldindata.org/grapher/cancer-deaths-by-age
[2] https://ourworldindata.org/grapher/share-of-population-with-...
[3] Rounded to the nearest whole number
[4] https://ourworldindata.org/grapher/life-expectancy
[5] https://ourworldindata.org/grapher/happiness-cantril-ladder