This may very well be a dumb question, but in an age where music singles can actually be defined as "single pieces of music for sale" and can be easily sold and downloaded online, how did Candle in the Wind get so popular when it was released?
I understand the cultural significance of the song itself and the context of its release, but all I can think is that 1997 was past the era of singles on vinyl (which makes sense for 1 song to take up one side of a small size 45" record) but too early for digital download singles. So all there is is cassette tapes and CDs. But when I think "I want to release a single" I don't think of those two mediums. Cassettes and CDs have the capacity for entire albums, so in my mind it seems like a waste of money/space to make a cassette or CD with only one/maybe two songs on it.
Or an I completely wrong and that is exactly what happened and tens of millions of people simply went out and bought this song as a single on a CD or cassette because that was just the norm for the music Industry at that time and that's just how powerful Princess Diana's death was? Again, perhaps a dumb question, but I just have a hard time understanding how music singles could survive the post-vinyl pre-digital era when CDs and cassettes don't appear to be conducive to the concept of singles, much less produce one of the best-selling ones of all time. Thank you in advance.
As you say, 1997 was past the era of vinyl 7" singles (which ceased to be a significant commercial force in 1990-1992), and before the era of the iTunes music store or Spotify. Instead, singles in this era were released on cassette tapes and in CD format, using CDs and cassettes that physically took up as much room as the cassettes and CDs that held full albums (as odd as that might seem today). There have been 3" CD singles as opposed to the 4.7" CD albums, most prominently in the late 1980s/early 1990s, but that format failed because not all CD players were capable of playing them because of how the CDs sat in the players.
These 4.7" CD and cassette singles were often distinguished from the albums by their packaging and price. Typically, CD singles came in either a) thinline packages with a small 'J-card' paper insert, or b) a cardboard case, as opposed to the larger jewel cases that full albums typically came in. Additionally, cassette singles often came in a rectangular cardboard box rather than the plastic case that cassettes typically came in. The website discogs has images of the packaging of the UK 'Candle In The Wind 1997' CD single and the 'Candle In The Wind 1997' cassette single, if you click on 'more images', which shows the J-card of the thinline CD single, and the cassette packaging.
By 1997, cassette singles weren't much of a commercial force compared to CD singles (which had better sound quality, and by 1997 CD players were not much more expensive than cassette players), but they were still sometimes in production, especially for larger releases; in the US and UK, there were cassette single versions of the Spice Girls' 'Spice Up Your Life', but there was no US cassette single version for the Smashing Pumpkins' 'The End Is The Beginning Is The End' (from the soundtrack to the Batman And Robin film).
In a story about pre-release demand for the 'Candle In The Wind 1997' single September 1997 issue of Billboard, the Billboard writer predicts that worldwide sales of the song will eclipse 'Do They Know It's Christmas' by Band Aid, also a charity single (in the era of 7" singles).
But yes, in the 1990s, singles had less lavish packaging, and were priced much more cheaply than full albums. Billboard claim that both the CD single and cassette of 'Candle In The Wind 1997', in the US, were both priced at $3.49. In contrast, full-length CDs varied in price, but a 2009 AOL article lists the price of a new release CD album in 1999 as being $13.65. As you suspect, it costs a very similar amount to manufacture a CD single vs a CD album; the record companies were making a lot of profit on CDs in the 1990s in comparison to vinyl records in the 1980s. So people were - in general - buying CD singles because they only wanted the one song, and didn't want to pay an extra 10 dollars for an album, similar to how and why they might buy a single song rather than a whole album on iTunes (except that there was less choice about the single song you could buy than there is on the iTunes music store).
Because of these commercial realities in 1997, No Doubt's record company deliberately didn't release 'Don't Speak' in CD or cassette single format in the US, despite its success on commercial radio in 1997 (it topped Billboard's airplay chart). No Doubt's record company believed that, in not releasing a CD single costing ~$3-4, they could direct enough consumers to the album costing ~$13-14, and thus make more profit (the song went to #1 in the UK and Australia, where it was released on CD single format).