About 8x10 and 4x5 in view of 5x7
Things could have been different!

The main virtues of LF compared to 35 and middle format are cameras with movements and high picture quality. 8x10 quite expensive and clumsy to work with compared to alternatives like 5x7 and 4x5; thus few cared to work with 8x10. Now, 4x5 ok to work with, but middle format cameras (normally without movements) much easier to work with, and cheaper, and picture quality not very much worse than 4x5; thus few cared about 4x5 compared to middle format and in view of a better LF alternative: 5x7 have movements and potential for extremely high picture quality, not as expensive and clumsy as 8x10 and roughly as easy to work with as 4x5. Thus, most LF fans choose to work with 5x7, lots of equipment and film available and 5x7 enlargers not very expensive and not very hard to get!
So, why aren’t things different?

Summicron1 and Roger Cole may be right in their analysis: (in free interpretation) things aren’t different due to historical coincidence, and given that things are as they are, not enough incentives to change.

For some reason 5x7 seems to be, or has been, more popular in Europe than in US; and AFAIK only European companies make 5x7 film today (Ilford, Foma, Adox/Efke). The popularity of 8x10 in US is perhaps due to American photographic icons like Adams, Weston, Evens et al (?))