"le" used to be the most common article because it was the most generic. While the most simple meaning of le- "something which the speaker describes as being one or more things that can be the x1 of [broda]" and lo- "something which actually is one or more things that is/are the x1 of [broda]", before the adoption of the "xorlo" proposal, lo had the additional restriction that what is being described must not only actually be a [broda] x1, but that it must also exist. So, for instance, to say "pa lo re gerku" would not merely state that you are talking about a single dog of two in total, but also that in the entirety of existence, there are only two dogs. Also, one can not refer to mythological creatures, such as unicorns, because at no time and in no place, so far as we are aware, at any rate, do unicorns exist. As you can see, this made the pre-xorlo definition of lo rather problematic, and because of this, it was seldom used.
Since the adoption of xorlo, however, all of the problematic portion of the meaning of lo has been dropped. The simplest way to show the difference between lo and le I can think of off the top of my head is this:
I have a dog which "thinks" it is a human. (By thinks, I mean it displays human mannerisms such as (attempting to) walk on two legs, eat at the table, use the toilet, take showers, put on clothes, etc.)
Because this dog is a dog, one cannot refer to the dog as "lo remna", however, one is able to refer to the dog as "le remna", because you aren't saying the dog /is/ a human, you are merely describing the dog as one due to the dog's human-like behaviours.