Why do some network connections dominate others?

Generally, the fastest connection wins. However, before Windows 7 networking was a mess and performance was awful all around. Try to get more than 20mb/s from XP good luck with that.

http://research.microsoft.com/pubs/70189/tr-2005-86.pdf

The standard TCP congestion avoidance algorithm employs an additive increase and multiplicative decrease (AIMD) scheme. When there is no packet loss detected (by means of three duplicate-ACKs or retransmission timeout), the congestion window (cwnd) is increased by one Maximum Segment Size (MSS) every RTT. Otherwise, if a packet loss is detected, the TCP sender decreases cwnd by half. In a high-speed and long delay network, it requires a very large window, e.g. thousands of packets, to fully utilize the link capacity. Therefore, it will take the standard TCP many RTTs to recover the sending rate upon a single loss event. Moreover, it is well-known now that the average TCP congestion window is inversely proportional to the square root of the packet loss rate.

A faster connection has more successful packets and therefore its cwnd/mss is increased and it gets even more of the total connection.