It's largely the same reason why NASA takes so long to do anything.
1. Shortage of funding
2. failure can result in loss/exhaustion of funding
3. extremely low risk tolerance
4. physical experiments needing new HW only happen when the likelihood of success is extremely high
5. projects are over engineered to reduce chance of failure
6. projects are over budget and over schedule
7. projects only make minor incremental progress
8. lack of fast/exciting progress drives decrease in funding
9. GOTO 1