When CTAN was established, most people would start using TeX with a 300 dots-per-inch (dpi) laser printer, and sets of Computer Modern bitmap fonts for this resolution are available on CTAN: fonts/cm/pk/pk300.zip (for write-black printer engines) and fonts/cm/pk/pk300w.zip (for write-white engines).
At that time, there were regular requests that CTAN should hold a wider range of resolutions, but they were resisted for two reasons:
If your output is to a PostScript-capable device, it may be worth switching to Type 1 versions of the CM fonts. Two free versions are currently available; the older (bakoma) is somewhat less well produced than the bluesky fonts, which were originally professionally produced and sold, but were then donated to the public domain by their originators Y&Y and Bluesky Research, in association with the AMS). Unfortunately, the coverage of the sets is slightly different, but the present author hasn't found the need to use bakoma since bluesky became available. In recent years, several other 'Metafont' fonts have become available in Type 1 format; it's common never to find the need of generating bitmap fonts for any purpose other than previewing (see "previewing documents with Type 1 fonts").
The commercial font suppliers continue just to keep ahead of the free software movement, and provide Type 1 versions of the EC fonts, CM-style Cyrillic fonts, as well as a range of mathematical fonts to replace those in the CM family (see "choice of scalable fonts").