Why are there so many varieties of plants? Well the theory of evolution explains that very well. Small variations acted upon by environmental feedback over 3.7 billion years or so would be expected to produce a vast diversity.
I don't know about "very well". Certainly it explains it and certainly it explains it better than any god in the gaps argument. But "vast diversity" means "vastly unfit for most environments" whereas most of the oldest species and the simplest species are the most abundant and from an evolutionary standpoint the "fittest". Variation is only explained well once it is assumed. Not that this lends any credence to any anti-evolutionary argument, simply something I have been working on as long as I have fitness functions (mostly as a curiosity).