U-2: why no imitators?

If imitation is the sincerest form of flattery then the U-2 has strangely missed out on the accolades it was due. On the one hand it was the most successful ISR platform of all time, while the technology behind it was not particularly unobtainable, at least after its first couple of decades in service. Even today it proves more reliable and cost effective than the Global Hawk.

So why didn’t anyone else build comparable aircraft in the 60 years since the U-2 entered service?