Why do I have a slow download from steam

Download speed depends on a few things, primarily how you are connected to the internet. A connection through an Ethernet cable is typically faster than a wireless connection. It can also have to do with how fast your hard drive can write data, but there probably isn't much you can do about that. Also, having many programs running while you are downloading or things in the background like antivirus scans tend to slow it down as well. With Steam specifically, sometimes the servers get "overloaded" to some extent, meaning if say, many people are on steam, getting on to steam, or downloading the game you are downloading at the same time you are, then that will slow down the download speed.