Why Is It Called Windjammer?

Short answer: Because the crews of the early steamships thought that they jammed up the seaways and harbours.
Windjammer is not a term for a specific shiptype, but a derogatory slur originally used by steamship crews about their sailing ship competitors.
Sailing ships have right of way before engine ships and back then had a harder time entering port, which gave rise to the notion that windpropelled ships jammed up the traffic.
Like most slurs windjammer was later changed to have the meaning that the very large late sailing ships had such large sails that they jammed up the wind itself. A notion established by proponents of sails.
Windjammer became a term used for all the very large sailing ships that competed with steamships and later engine ships.
It is an irony of fate that the windjammers ended up lasting more or less as long as steamships in commercial use. Both being replaced by motorships with diesel engines.