Why upload rates are lower than download rates in normal internet connections?

Most broadband services use ADSL: asymmetric digital subscriber line.

The "asymmetric" bit of that refers to the asymmetric bandwidth, i.e. the upload bandwidth isn't the same as the download bandwidth.

From the referenced wikipedia article:

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.

I think the majority comes down to the fact that the main part of people using internet connections will need more download rather than upload.

For the most part we use upload to tell servers what we want (HTTP protocol as an example) and through download is where the heavy lifting comes.

The thing is, that if you run a server then you want a good upload - but most of the time it will be companies running servers - so they get to pay extra for this - and it's a brilliant way to charge extra where it can be done (mainly this is done by selling them a completely different technology though).

Of course there's the limitation of ADSL as well - and the ISPs want to give their customers the best experience, which is done through good download and not too good upload.

All in all a certain ratio needs to be kept for the massive download to be worth anything - and normally (at least in Denmark where I live) you get a little higher upload as you get better download.