Why is download speed not increased using an SSD? I upgraded my hard drive from HDD to SSD.

There is no change in download speed. Reviewer says SSD has very good write speed, but why does that not increase download speed?
Probably the bottleneck is not your disk but your network.

A SATA2 hdd disk usually supports write speeds of up to 250/300MB/s. It is unlikely that you can reach those download speeds.