Probably nobody knows any explanation. A lot of things in Bitcoin are still holdovers from when Satoshi wrote things just by himself, and he often failed to explain the reasons behind what he did. Probably it was yet another anti-spam system. In theory if we set the soft limit to 1mb and then all the space got filled up with useless spammy transactions it could be easily lowered again, but trying to find the balance between "good" and "bad" transactions is difficult. Especially because when we don't find a block for a while the good transactions pile up.
At the moment we don't seem to have much of a spam problem, so it makes sense to open up the throttle. Potentially, a soft limit phrased in terms of transactions per minute would make more sense than one based on a fixed block size.
That's like throwing away your umbrella in a rainstorm because your not getting wet.
The day will come when the block chain is so large and its acceptance so wide spread that just maintaining a full copy of it will max out all consumer level internet connections.
Postponing that day for as long as possible is the single most important Bitcoin related issue.
