Why adsl upload speeds slower than download speeds?

This is by design, and is reflected in the name - the A in ADSL stands for Asymmetric, reflecting the fact that the speed is not the same in both directions.

There are both technical and marketing reasons for this. On the technical side, it is simpler to provide high speed in the download than the upload direction. On the marketing side, this allows companies to sell high download speeds to private customers for a relatively low cost, since download speed is what most households are interested in, while at the same time charging much more for business customers that need high upload speed as well.

See related link.