There are probably a few reasons. I can only speculate.
The weapon needs estate to mount Laser for night fighting and light, as well as the ability to mount GL. The legacy systems like AUG and FAMAS are terrible for mounting GL and additional gadgets.
They are also too heavy for what they are, especially FAMAS and SA80. AUG also has issues with water and other mechanical issue. Hence, all the special forces guys ( and those who can get away with their own weapon procurement projects ) ended up buying M16 back in the days, and now HK 416 or M4 to replace them.
Of course, the specical forces people are the first to get the high end gadgets such as laser pointers, lights, clip on thermal devices....etc. and these all need rail space. The US is the biggest origin of these gadgets because they have the biggest buyers. So all the items are designed around mounting on the M16 FOW.
When the next upgrade or weapon purchase comes, the spec op experience probably gets incorporated and the people in charge also look at how to include future/existing gadgets to the system. Also, the regular joes see the cool guys running around with HK416 and M4, they wonder why they get stuck with a different weapon system and that sentiment got funneled upwards, ie, the cool guys know better and get the better weapons. Well, the M4 and the HK416 ( and now SCAR ) are indeed better than all the bullpups out there.
The truth is that if you are running an European army, you can only go for the new Thales AUG or the AUG A3, an old system with new packaging. Outside of Europe, add the IWI TAVOR or X95. Most of the countries outside of Europe are jumping from G3, FAL, AK or M16 to something else. A lot of them probably want to stick with the conventional format for formality. That is probably the reason why we see some countries are hopping between the TAVOR and the Galil ACE, with their bullpup experiments ( and depending who it is charge for that department at that moment).
