A good reason for an orchestra to be slightly sharp is that it will produce a different sound. Not because there is anything special about the frequency 442, but simply because listeners are used to hearing concert pitch everywhere. Although most people do not have absolute pitch in that they can tell that a given orchestra is sharp by a few cents, they can probably still sense that something is different. The frequencies in the music are not hitting quite the same spots on the basiliar membrane of the inner ear as music in A = 440.
Part of it could simply be artistic defiance: We're not going to let some standards body tell us what pitch to play! What for? This music is a final product. If it is recorded, it's not meant to be a backing track for someone to jam with, so why does it have to conform to some absolute pitch? Standards are for things like nuts and bolts, not for art. No, siree. We are going to take the standard and "put our two Herz in".
It could also be snobbery to some extent: keeping low-end, untuneable instruments out of the orchestra. Not only that, but also preventing the "people at home" who listen to recorded music from being able to play along with the orchestra and actually sound good. An amateur trying to play along with the music will be slightly flat, which instantly creates the false sense that something is wrong with his or her skill relative to the musicians in the recording. The purpose of the tuning standard is that anyone can instantly play along with anyone else, or with a recording of anyone else. Snooty musicians have obvious reasons for rejecting such a concept.
It should be noted that instruments do not always have perfect mathematical intonation anyway. For instance, on a piano, the upper end of the keyboard is tuned a little sharp and the lower end is a little flat. So even if the A above middle C is 440, the A which is several octaves higher will not be an exact multiple of 440.
This is because the harmonics themselves are not perfect multiples, and pianos are tuned so that the higher notes of chords do not jar against the harmonics of the lower notes.
In a mathematically ideal string, the harmonics are perfect multiples, but in real strings, the harmonics are a little sharp. So a piano's A=440 note will not have a harmonic that is exactly 4 x 440 = 1760 Hz.
This means that pianos are rarely "in concert pitch" all across the keyboard, and any piano which is will sound worse for it. If a piano isn't in the same pitch throughout its range, it is implausible to think that an entire orchestra is.