Page "FAST TCP" Paragraph 4

Most current congestion control algorithms detect congestion and slow down when they discover that packets are being dropped, so that the average sending rate depends on the loss probability.

This has two drawbacks.

First, low loss probabilities are required to sustain high data rates ; in the case of TCP Reno, very low loss probabilities are required, but even new congestion avoidance algorithms such as H-TCP, BIC TCP and HSTCP require loss rates lower than those provided by most wireless wide area networks.

Moreover, packet loss only provides a single bit of information about the congestion level, whereas delay is a continuous quantity and in principle provides more information about the network.