This is largely due to a combination of two different factors:
Some of our uploaders (such as myself), do not have a very fast upload speed, while some of the leechers have an amazing upload speed. In the case of uploaders such as myself who don't have a decent upload speed, it is only possible for me to upload any of the parts of the file to any given user at a slow rate (usually about 45KB/s). When there are one (or more) leechers who can crank out speeds of say 1MB/s and there are several leechers, eventually those leechers with the quick upload speeds will help get other users caught up to where they are in the process as the torrent traffic tends to favour the fastest connection, generally preferring active and connectable nodes, often ignoring the passive ones that use follow-back packets.
In light of the last portion of #1, there are a large number of users who are running behind a NAT, which is most often found built right into their broadband modem to masquerade a /24 subnet (say 192.168.1.0/24) issued to each computer connected to the modem's built in router behind a single IP address visible from the internet. This is not necessarily a problem though, if the NAT is configured correctly. In my case, my torrent program listens for incoming connections on port 22791. When my computer announces to the internet through the NAT, it can get an outbound connection no problem to announce that it's listening on port 22791. If you do not program the NAT in the router to forward inbound connections from the WAN (Internet) on TCP & UDP 22791 through to my laptop in some way, the NAT will drop them by default because it does not know where to route them to, since it does not use these ports itself for anything. The result is that you end up becoming a passive node. More often than not, passive nodes are unable to upload if they have SPI/Firewall protection enabled on their router and have not properly confiugred the port forwarding in the NAT. Some connectivity will be lost to discovering the passive nodes and sniffing out active ones to take preference over.