The Clickteam Blog mentions that the Fusion 3 image editor is tracked for being "something along the lines of which we’d had for years already", as well as better third party support (opening sprites directly in the app you choose rather than F3's editor, according to the blog post). I'm no professional artist but for pixel-based stuff the built-in editor in Fusion 2.5 has served me well enough, and anything more advanced that I need I use GraphicsGale/Affinity Photo/another editor etc
Clickteam is a pretty small company, and unlike many other game engines the cost of it (+extensions) is a one-off purchase - no licencing fees, no revenue sharing etc. They've also just released their biggest extension yet since the launch of F2.5. Yes, some enhancements like multi-selections etc in the image editor would be nice, but I imagine most of us would rather CT continue working on Fusion 3.0 and enhancements to 2.5/2.5+ than spending time on an image editor that already does the basics, especially when there are many low-cost or free options for image editing that fill more specific needs.