Because you can write code in ActionScript3.0 and compile into SWF file using any edit program and any Operating System.
Also you are not bound to buy a license for flex 4 or Flash Profesional CS 3,4,5, a license which cost a lot for any middle person.
I am using at this moment Ubuntu Linux v.10.10.
1. Download Flex 4 SDK from this link (Download -> Free Adobe Flex SDK) or directly from here . Save it to Desktop.
2. Unzip and change the name to "flex"
3. Open a terminal (to use the command line) and write the following code to create a folder called "flex4" at address "/opt/"
sudo mkdir /opt/flex4
4. Now, you need to move SDK program from Desktop to that address . Here is the code :
sudo mv flex/* /opt/flex4/
This command move all files from flex to flex4 folder. If you want to check you moved everything , you can write in terminal :
cd /opt/flex4
ls
5. Now, your machine needs to know what to do when you type in terminal a code like
mxmlc file_name.as
This command line compile the code wrote in file "file_name" in a SWF file.To know, you need to add the following code in a specific file called ".bashrc" :
gedit ~/.bashrc
and add(copy -paste) to the last line
export PATH=/opt/flex4/bin:$PATH
Have fun , programming is fun!