Why are upload speeds slower than download speeds?

Upload speeds are generally slower than download speeds because of the fact that most internet connected equipment is asymmetric. Asymmetric means that any one packet of data can either go in, or out, but not both ways at the same time.

Since most internet users are receiving more data than sending, the internet connected equipment has shifted the higher majority of the signals to go toward downloading.

If an internet provider allows your connection to be 5Mb download and 1Mb upload, this same connection is capable of handling 3Mb upload and 3 Mb download. But this would then slow down the most important part of the internet, the receiving and downloading of requested data.