ADSL stand for Asymmetrical DSL (or if you wish, Asymmetric digital subscriber line). In this type of communications speed is always considerably higher in one direction, hence the ""Asymmetrical"". However, on a Asymmetrical connection, which direction is higher is determined by the service provider. So there is not real impediment for upload speeds to be higher than download.
As for the reasons why this is a preferably mechanism, read below (source):
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL. On the marketing side, limiting upload speeds limits the attractiveness of this service to business customers, often causing them to purchase higher cost Leased line services instead. In this fashion, it segments the digital communications market between business and home users."
"many ISP offer a ratio of 8/1 for domestic connections (download speed is 8 times higher than the upload speed). and as usual, the small print applies, it's always 'up to'.
contention ratio is another factor for varying connection speeds, the higher the contention ratio, the greater the number of users that may be trying to use the actual bandwidth at any one time and, therefore, the lower the effective bandwidth offered, especially at peak times.
for a more accurate answer, refer to to the connection specifications of your provider."