Hummingbirds tend to hover in front of a flower or feeder prior to committing to it. Mantids are ambush hunters, so they lie in wait in those areas. I've never seen it myself but I imagine they mostly snag the hummingbirds while they hover in place or wait for them to land, which would mean they don't have to worry quite so much about getting ripped off their perch. Hummingbirds may be physically larger, but they only weigh about as much as a female praying mantis. They have been known to catch bigger and heavier animals, though.