It has to with handling the grapes and how it is processed:
Answer: L’Chaim, David! Answering this question requires a nice glass of vino. You don’t mind if I type and sip, do you?
In order for wine to be kosher, of course it has to contain only kosher ingredients. And according to traditional Jewish law, once the grapes are picked and brought to be crushed, only Shabbat-observant Jews can be involved in making the wine. From crushing to bottling, kosher wine must be handled exclusively by observant Jews.
Why the strict rules about only Jews? Because in the past wine was often used by pagans in their offerings to idol gods. When something good happened, you’d pour some wine out on the ground as a symbolic thank you (if you were an idol worshipper, that is). The rabbis who set up the rules for kosher wine wanted to make sure that Jews never got a glass of wine that had been associated with an idolatrous offering, so they required that only Jews be involved in handling kosher wine.
Even after these rules were set up, some people worried that if you had a nice glass of kosher Chardonnay at a Jewish wedding, it’s possible that the non-Jewish waiter or waitress might have spilled some of your Chardonnay in an idolatrous practice, while your back was turned. The solution: Mevushal wine. (Shulhan Arukh, YD 123)
Mevushal (literally “cooked”) wine has been heated to the point that idol worshippers wouldn’t use it for their nefarious purposes. It turns out even idol worshippers had standards for their wine. They wouldn’t use wine for an offering if it had been boiled because boiling wine removes much of the flavor. So the rabbis ruled that in order to avoid the possibility of a Jew ever drinking wine that was idolatry-associated, only cooked wine could be served to a Jew by a non-Jew.
Today, people don’t do a lot of pouring wine out for the gods. Still, because of the previous rulings by various halakhic authorities, some people are uncomfortable with a non-Jew pouring them a glass of kosher wine. So mevushal wine is often served at events where non-Jews will be doing the pouring and serving of wine. This stance, of only serving mevushal wine when non-Jews will be serving, is the norm among Orthodox Jews, and those who follow the regulations of the Conservative Movement‘s Committee on Jewish Law and Standards.