@stefl1504
It's pretty simple really.
When the PSP released (hell even with the PS3 released) the PSN wasn't as developed as it is now and thus, for PSP games back in the day to have online (as opposed to ad-hoc), the developer needed to bear all the load.
What I mean by that is they need to think of what packets to send, what ID to have people use, run servers, maintain those servers, how to deal with authentication, etc. It's kind of a pain.
With the PSN in full swing now I believe Sony gives devs some standard APIs for use for authentication and we now all have a ID we can use for all games. This stuff just didn't exist for the PSN in 2005. So, we can only get "real" online via the infrastructure mode which is more stressful to devs and probably not worth it for most of them.