JBoss projects moved to JBoss owned implementations for a few reasons. I can remember the following:

• being able to fix bugs for users and customers
• licensing is a bit unclear on the javax.* (they were not under an OSS license, don't know now) so we wrote our own
• during Java EE implementation cycles, Oracle is late in releasing intermediary artefacts so we rolled our own
• some JAR have specific code to prefer the container impl as opposed to the first one found, so controlling that is nice