Why is a derailleur guard present only on more affordable bicycles?
Derailleur guards are rarely found on more expensive bikes for several reasons:


Protection: These guards only protect from minor contacts. Forces that would bend a typical derailleur would also bend these guards. Also, more expensive bikes usually have exchangeable derailleur hangers that are supposed to be the first thing damaged in a crash (and, thus, prevent the more expensive derailleur from more damage).
Weight: if the bike is handled properly (as owners of more expensive bikes tend do), the little added protection against minor touches is generally seen as not necessary.
Style: These things are ugly!