Silly Question about "The Big 3"

[Life Long Learning]

I was just wondering why TESU, EC and COSC are considered "The Big 3"?  

And why isn't WGU included in this group to make it "The Big 4", due to it's exploding popularity by online degree-seekers over the last couple years?

Already answered- but it's all about transfer limit.  
When I started my degree process in 2006, you could still transfer in 100% of your credit, and those 3 colleges were the only ones in that category.  Now, you can transfer in almost all (117? I don't even know) so it's not that the category is for online degree-seekers, it's more about credit-shopping to craft a degree completely DIY style.  At present, you can use so many different types of credit (credit by exam, ACE, college transfer of old or new credit, PLA, etc.) that there is something for everyone in terms of how to get the job done.  In fact, to be technical, you can fill your Big 3 degree with brick and mortar classes- they don't even have to be online.

When the Big 3 started out, it was 99% putting brick and mortar credits into a degree program for folks who fell through the cracks by the “my way or the highway” B&M Universities.  The explosion of online options and credits has made credit shopping a DIY skill.