I think they did that because of how well it worked for GMail. And Facebook was restricted too in the beginning. Obviously, Google+ didn't benefit from following the precedents set by other successful launches.
Compared to that, nobody cared about a Facebook ripoff with a different UI and none of the network effects.
https://en.wikipedia.org/wiki/History_of_Gmail
"after considering alternatives such as 100 MB, the company finally settled upon 1 GB of space, a figure that was preposterous compared to the 2 to 4 MB that was the standard at the time.[3]"
"On 1 April 2005, exactly one year after the initial release, Gmail increased the mailbox size to 2 GB, advertising it as 2GB plus"
The sample of the news from these times:
https://betanews.com/2005/04/01/gmail-storage-to-reach-2gb-a...
FB wasn't restricted within your likely friends group (all students at particular schools were made eligible at the same time).
1. Exponentially more space in a market where single or double digit megabytes was the norm. GMail's original pitch was that you never had to delete an email, and that you could always find them with search.
2. It was one of the first web apps. It was dynamic in a way few sites were then. Asynchronous JavaScript (Ajax) and Web 2.0 were still new concepts.