The big players oppose JDO

J2EE 1.5 is meant to focus on developers, particularly on 
 making development easier and the platform easier to use. That has brought the question 
 of a data persistence layer to the fore.

The likely course is to dust off EJB and give it a new incarnation 
 numbered 3.0, which Pavel Kolesnikov reported on
 in his posts EJB 3.0 revealed for the first time!
 and Entity beans v EJB 3.0.

What’s interesting is that EJB’s heaviness and appetite for system resources were its main problems. That’s why lighter
 frameworks sprang up, such as Hibernate. Another step was
 the adoption of JDO(Java Data Objects) as another standard 
 for the persistence layer.

J2EE 1.5 and the associated EJB 3.0 are precisely why IBM, BEA and Oracle took
 a negative stance on JDO 2.0, as they showed when voting on JSR #243 Java Data Objects 2.0 — An Extension to the JDO specification
 JSR Review Ballot. That vote sparked a big discussion on TheServerSide in the thread 
 JDO 2 Ballot Results: Concerns from IBM, BEA, and Oracle.