Why isn't the F-15E called F/A-15?
The aircraft you are mentioning are completely different from each other and some of them are not even named according to the US Tri service Naming System.

In general, the aircraft retain their original designation given even though the later versions have little in common with their predecessors.

The F 15 was a direct result of the studies that followed the Vietnam war and the a response to new generation Soviet aircraft like the Mig-25. 



"McDonnell Douglas F-15A USAF". Licensed under Public Domain via Wikimedia Commons.

The F-15 was designed purely as a air superiority fighter, with full emphasis on maneuverability and agility with "Not a pound for air-to ground!". The first to variants of the F-15 were called F-15 (single seat) and TF-15 (two seat), which were retrospectively named F-15A nd F-15B after the introduction of F-15C. The F-15E was a strike variant; however, it could defend itself against enemy aircraft and thus retained the 'F' title.



"F-15E Strike Eagle" by Gerry Metzler - IMG_214. Licensed under CC BY-SA 2.0 via Commons.

The case of F/A-18 is different. The initial plan was to purchase three different version of the aircraft, F-18A (Fighter), A-18A (Attack) and TF-18A (Trainer). However developments in avionics and some design changes led to the 
first two versions joined together as F/A-18A and the trainer version being named F/A-18B.



"US Navy 050803-N-0295M-148 An F-A-18B Hornet, assigned to the U.S. Naval Test Pilot School, folds-up its landing gear after taking off for a training flight from Naval Air Station Patuxent River, Md" by U.S. Navy photo by Photographer’s Mate 2nd Class Daniel J. McLain 

Most of the aircraft in service today are multirole and their names mostly reflect what their intended purpose is rather than their actual purpose. With modern avionics and high precision weapon systems, almost any aircraft could be made a strike aircraft

It is to be noted that the naming convention is followed only loosely in some cases. A good example is the F-117 Nighthawk, which is not a fighter aircraft in any sense of the word.