The biggest thing to understand about an Orchestra is that they operate "at a loss".
To have a successful orchestra, you need to be able to have donors, ticket sales, and subscriptions.
Donors make up the most of the monetary substance. Subscriptions come in second, and ticket sales dead last.
On the other end (money out), you need to pay staff, musicians, and such for time to practice, perform, and hone their skills. Being a professional orchestra level musician is a life style more then anything else.
So now you have a large money in, large money out "business" that makes no money itself. Most orchestras struggle just to "survive", but even "thriving", can mean just breaking even.
So, the question becomes, what do you need to be able to play the most varied selection of music? Would you donate $10,000 to an orchestra that couldn't play "Beethoven's 5"? How about one that couldn't play Opening theme from Star-Wars? What about something like Vivaldi's Four Seasons?
Ok so now you have a pretty set list of players you need to pay. You have your strings, your percussion, your brass, and you wood-winds.
Now, be need a Trumpet player. Guess what there's a lot of those so we can afford 4 trumpets at around $70k each. The better ones more, the less skilled less, but averaging around $70k.
We should have some french horns. Hmm there staring around $100k on the low end. Arrg. Ok let's get 1.
Now lets get an oboe player. Lots of those again, They start around $62k. Lets get 4.
You know what. I want to play a piece from Zelda, Link's awakening. I need an ocarina player. Hmm, well theres only 3 of those in the whole world, and the only one that wants a full time gig wants $1,200k a year. Hmmm, let's re-write the ocarina part for clarinet. WOOT $36k a year. Still only need one, but wow was that better then $1,200k.
And so on.
Most pieces call for a similar set of instruments. Pieces that call for "special" instruments can be arranged to not need them. Thus I don't have to pay for a full year just to play 1 piece.
If this year's schedule calls for 30 different pieces, and they all have the same instrumentation, that's great. If 29 pieces have the same instrumentation, and 1 piece needs something special, then maybe I should rethink that one piece.
In the real world, most pieces have mostly the same instrumentation. It's generally close enough to pay for the 4th trumpet to just play the 3rd part with the 3rd instrument on the 4 pieces that only have 3 parts. But an odd/new instrument that's only around for one piece? What are they supposed to do the rest of the time?
This is even more true if the piece the new instrument plays is part of a concert that is only given once or twice a year.
Including this player in the orchestra becomes a waste of money. Better to just contract them out for that one performance. However doing so could be very expensive.
So while an orchestra may not actively avoid having new instruments. It also doesn't make a lot of sense to have a player on the books that isn't going to perform in many pieces. Better to have them as a "guest" then a "member".
That said, most orchestras are more then willing to have "new" instruments that are replacements for old ones. But an electric violin, doesn't sound the same as an "acoustic" one. Sound is everything in music, so....
Would you want to listen to a MIDI keyboard over a Grand Piano?
No, totally new instruments simply wouldn't get enough playtime to be worth the cost, while new version of old instruments are more accepted, they also don't sound the same, and that's important. Combined, it means that orchestra companies tend to favor the same (rigid) set of instruments.