There certainly isn't one single reason, and, yes, “that's just how it's always been” is probably an important point.
However, I would quite agree that guitars should not be part of a standard orchestra: for the same reason pianos aren't, and shouldn't be, part of an orchestra.
Orchestral music takes a lot of its attractiveness from the art to weave individual, expressive melodies into a whole that's a whole lot more than the individual melodies. Themes keep wandering through the various instrument sections, and each permutation sounds different but can be used to introduce counterpoint in an analogous way. There aren't really any accompaniment instruments as such.
For this to work properly,
- every instrument needs to be able to make very fine adjustments to its dynamic level throughout the performance.
- It needs to be able to produce a loud yet smooth sound to cut through the ensemble and reverb, without itself disrupting the balance. Percussion is a bit of an exception here (but in symphonic works, percussion is usually more of a “special effect”).
- And it needs to be able to perfectly “lock” its intonation, timing and phrasing to the rest of the orchestra.
Acoustic guitar pretty much fails in (2.) and, due to limited tone control, also has trouble with (3.). Electric guitar is risky in (1.) and (2.), and phrasing and intonation tends to come out pretty different from orchestral instruments – a problem you also have with piano. So these instruments don't really blend into the orchestra.
Of course that doesn't mean you can't use guitars or piano with an orchestra.
- In a solo concerto, it's the orchestra's responsibility to adjust to whatever sound the solo instrument produces. The solo voice is not intended to blend in at all, but in fact stand out clearly.
- The opposite: e.g. in baroque music, there is usually a chord accompaniment that's subordinate to the melody instruments, much in the same way as rhythm guitars in pop/rock music.
But neither of these options is really in the spirit of symphony, with its, as it were, “communist” hierarchy.