To get around this limitation, make use of the Windows Metafile format. Here's how...
1. In Visio, select the page containing your diagram.
2. Click File > Save As and save the diagram as a Windows Metafile (wmf).
3. In your PowerPoint file, click Insert ribbon tab and select Picture
4. Browse to the folder containing the wmf file saved in step 2 and select it. The image will now show up on the slide.
5. Select the image and go to Format ribbon tab. Select Group > Ungroup option.
6. Now if you click at any place on the image, you will be able to edit the text or make changes to the shapes in it.
This does not give you a lot of control on the editing, but it is definitely a good work around if you ever need to share your Visio diagrams in a PowerPoint file and keep the text in the diagram editable.