Because they (delete as appropriate)

• are here.
• fit best the company requirements
• are easy to remember
• have best potential to scale

But I have faq question as well - why users like to start discussion in inappropriate "Additional Communities" forum instead in an appropriate one ?

P.S. Don't forget to rate the answers provided.

Hey Dude, 

Note that networks today use Classless addressing i.e., Class A, B, C is not used anymore, they arem more like a reference:
Companies now use classless private IP Addressing ranges:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255

You probably meant why companies use the "10.0.0.0 - 10.255.255.255" range right?

Well, based on what I asked our clients:
1. The other addresses e.g., 172.X.X.X and 192.168.X.X (especially the 192.168.X.X) is commonly used by a lot of companies, and would be easier to guess
2. It has more subnetworks, e.g., there is only one /16 network block in 192.168.X.X while 10.X.X.X has a lot of em. So if you have a very very very large network, then at least your network always start with 10.X.X.X
3. Aesthetics, since 10.X.X.X is not common to most users. Most users only know about 192.168.X.X so its ups up the geek level haha xD