Java is an older technology, and can be free if hosted on an Apache server (or other open source tools) or can be hosted by vendors such as IBM WebSphere or Sun Microsystems (inventor of Java).
ASP (ASP.NET specifically) is the same thing my site uses (ScoutData.Com) and it's a Microsoft solution. It's newer, they recently just released the latest 2008 editions of the development, database, and server environments. However, the Microsoft stuff can generally cost a great deal more (unless you are using the IBM Java solution), but being a non-profit, is difficult to speculate.
What is really interesting about this is not the technology, but what that change represents. Generally, you are a .NET developer or a Java developer (I assume multiple people work on the BSA infrastructure), however some have switched sides, but rarely concurrently. I'm curious if the original site was developed by an outside organization and managed by BSA (aka consulting/contracting), or perhaps is now outsourcing it and selected a Microsoft toolset.
Either way, being biased and preferring Microsoft development technology, I see this as an upgrade and glad to hear they are making the switch. Hopefully their development teams are in-house and this wasn't a major expense. Also if they need any help with MyScouting, my team would gladly volunteer, hehe.
Yours in Scouting,
Phillip Zedalis
Lead Developer - ScoutData.Com