I do not have concrete reason to mention why SAP came up with separation of ABAP and JAVA stack installation (unlike Dual stack installation performed earlier ).

It is evaluated and believed that performance was one of the major reason for it. As of SAP Business Suite 7, you can no longer install Dual Stack Application Systems (ABAP+Java) except for Solution manager and PI .Existing dual stack Application Systems are still supported.

However, new systems should be installed as separated ABAP and JAVA Systems.

If you want to use both SAP Business Suite ABAP and Java Components in your system landscape, you have to proceed as follows:
1. Install a separate ABAP-based SAP Business Suite system.
2. Install the required Java components in a separate Java-based SAP system.
3. Configure the connection of the Java-based SAP system to the ABAP-based SAP Business Suite back-end system.

EHP4 for SAP ERP 6.0, SAP CRM 7.0, SAP SCM 7.0, SAP PLM 7.0, SAP SRM 7.0 are part of suite 7.

There is some work around for BI for which one can refer to SAP note :-
Note 1181025 - NW7.0 EHP1 - Install BI Dual Stack System (Workaround)