ULers often replace heavy gear items with experience and skills, rather than just going out and buying a lighter version. For people who only get out for one trip a year, acquiring those skills may not seem possible. For example, a tarp instead of a tent, or a twig fire instead of a canister stove.
If someone is really into hiking, they'll get out and do it and learn what works for them--at their own pace. I love talking gear and technique with people who are genuinely interested and eager to learn, and I meet more and more of those people. For instance: I regularly drink out of a Platypus at work, which my students find totally intriguing--and just about everyone can appreciate the convenience of a bottle that collapses down to next-to-nothing when it's empty. "Whoa, it's like a reusable Capri Sun!" a kid once exclaimed to me.
A lot of people I meet who've been bitten by the UL bug are older folks facing the possibility of knee/ankle/hip surgery, back and shoulder problems, etc, who simply wouldn't be able to get out in the mountains if it weren't for UL gear. After being warned you'll never backpack again, it must feel great to get out and prove the medical "experts" wrong.
Younger folks are often resistant to UL gear for lots of reasons. There's an element of machismo to "proving" you can carry a heavy pack up a mountain. When you're young and your body repairs itself easily, you may not be aware of the damage you're doing until years later.
Moreover, many people are intimidated by the apparent cost of replacing gear, especially "Big 3" items; I carried the same pack from the time I was in high school til well after I was done with university. Luckily, it "only" weighed 3 lbs and fit well, but justifying a new pack was very hard for me until I had all but destroyed my first one. And if I didn't hike and travel regularly, it would have taken me a lot longer to destroy that Gregory. :) I'd love a 1 lb shelter, but it's not in the budget right now, so my 2 lb option will have to suffice.
The heart and soul of UL, however, isn't buying fancy gear; it's being able to look long and hard at what you take with you, what you take for granted, and ask yourself whether you really need it or not. I see people carrying 16 oz bottles of Dr Bronners, sunscreen, bug stuff, etc, on weekend trips. Putting your toiletries in mini bottles costs next to nothing!
In my experience, it isn't possible to "convert" people, and I'm not a fan of anyone who treats their beliefs like gospel that must be preached--especially when I'm in the mountains. But, if you're out with an 8 lb base weight and you are clearly safe and comfortable and happy (and good company on the trail), *that* makes a strong impression on other hikers, and is probably the best way to be a UL "ambassador".