Why are there so many variety of hosta?

Tonight we had a friend over for dinner, he is a regular at our house on Tuesday nights. He glanced over and saw my Hostapedia sitting on one our end tables. At first he asked me if it was a joke, but when he realized it was the real deal he glanced again at it. For those who don't know, the Hostapedia is 1125 pages long and weighs a little over 12 pounds. It's huge, and I'm surprised my end table hasn't collapsed under the shear girth of it. After his laughter cleared over my owning a Hostapedia he asked me a really good question. He asked me why there were so many varieties of hosta compared to other plants. I'm sure he doesn't know or care that daylily varieties are also numerous (and I'm sure other plants), but it also really made me think. Why is it that there are so many varieties of hosta? I am not a scientist, I am just a home gardener (with a hosta addiction). But here are some of the reasons why I think there are so many varieties of hosta.

1. Hosta do not come true from seed, so if you plant a hosta seed, it will likely not look like it's parent plant. So hostas create their own different varieties without human help.
2. Humans step in and purposely cross pollenate flowers to try to create new breeds of hosta. Even though you never know what you will get when you cross pollenate, it's easy to do.
3. Hostas will create their own sports.
4. The science of using tissue cultures to create even more new, unusual hosta varieties has progressed far over time.
5. Hostas are a very popular and easy to grow plant, therefore there are lots of collectors always looking for new varieties.

I know I am overlooking some obvious reasons, so why do you think there are so many varieties of hosta?