Many reasons. Lead-acid batteries have what is called a linear discharge curve. This is where the available voltage decreases with use. Second, as the charge is depleted, high loads cause the voltage to sag even further, rebounding only when the load is removed. These two conspire to make the usable energy for real work about half of the rated capacity. It also makes measuring remaining "fuel" an approximate guess at best. In so many words, they quit a lot sooner than you would think, and with less notice than you'd expect.
They also require very disciplined attention to charging. Get in a hurry and forget to plug in the charger after use even once or twice, and you shorten their life. Repeated deep discharging - more than 50% of capacity - also shortens life, considerably. Forgetting about 'em in the shed during off-season without a maintenance charger is fatal. "Life" in the case of most well-designed deep-cycle lead-acid batteries is roughly 300 discharge cycles, and that's with attentive treatment. IOW, they are consumed, and fail altogether a lot more than folks without hands-on experience realize.
Then there is all the weight you have to lug around.
Li-Ion solves almost all of that. The available power is surprisingly constant. You get about 80% of rated capacity, and then power falls off a cliff. Easier to predict, much longer useful work time. They are much, much lighter per contained power. Not weightless, obviously, but roughly 1/4 the weight of the same capacity in lead-acid. More tolerant of poor attention to recharging, with nearly all systems capable of 1000-1200 discharge cycles.
I'm not regurgitating marketing hype... I've owned an "old school" electric car for 18 years and know all of this the (very) hard way. It turns out that I would have been better off by just buying a cheap subcompact in the beginning, because the replacement cost of batteries (on my fourth set now) amounted to the gasoline cost of something that gets 16-18 mpg. It was false economy, even when gas was $4.50 a gallon. The early electric car fan-boys weren't telling the whole story. Modern electric cars with their high-tech motor control systems and expensive Li-Ion batteries don't exactly break the lifetime cost barrier, but at least they're a heckuva lot closer to break-even.
There has been lead-acid powered "cordless" lawn equipment around for several years, mostly small push mowers. They didn't catch on, for good reason. I've also recently seen a couple of lead-acid-based small riders; Home Depot is currently clearing out a couple of models. The larger version uses the same batteries as in my car, and I wouldn't touch it with a 10-foot pole for that reason alone; been there, have the t-shirt.
Program Note: I write this doctoral thesis (ha!) as I am rushing out the door and will be away from the computer for several days. I hate it, too, when somebody drops a bomb like this and then disappears. Please accept my apologies for not being able to respond to any questions or comments 'til we're back.