Hi J.,
I've been reading up on this because I was wondering the same thing myself. What I've come across suggests that many kids aren't ready for a toddler bed until they are older then 2 and even closer to 3. Most kids will try to climb out before then, but the 'experts' recommend getting one of those crib nets to keep them in.
The REASON they give is that many kids just aren't ready for the big boy/girl bed sooner. They don't understand the imaginary boundary around a bed and climb out, or are so active in the night they roll off. Older kids and adults typically don't roll out of bed often. Our subconscious understands the imaginary boundary around a bed. Small children don't.
So, apparently it is better to wait as long as possible.