Why don't we have a better telescope than the Hubble Space Telescope? The Hubble Space Telescope (HST) was launched in 1990, more than 20 years ago, but I know that it was supposed to be launched in 1986, 24 years ago. Since it only took 66 years from the fist plane to the first man on the Moon why don't we have a better telescope in space after 24 years?
 A: The key reason why there will never be a next-generation optical telescope in space is: adaptive optics. 
In 1978 when funding for the HST started, adaptive optics was in its infancy. Only in the 1990's when computer technology had sufficiently advanced, did adaptive optics really take off. Modern earth-bound telescopes with adaptive optics are far more capable than anything we can bring into orbit at comparable costs.
To give you an idea: the European Extremely Large Telescope (E-ELT) sports a 1000 square meter light gathering area, and will be build and operated for a very tiny fraction of the 10+ billion USD total price tag of the HST (with a mere 4.5 square meter light gathering area). 
It simply makes no sense anymore to send optical telescopes in space. This is different for telescopes operating at non-optical wavelengths for which the atmosphere has a low transmissibility. COBE, WMAP and Planck all operate at microwave wavelengths, Chandra at X-ray wavelengths, and the JWST is designed for infrared wavelengths, 
A: A big reason is that through subsidized servicing missions the Hubble has been substantially upgraded over the years. Very few of the original HST instruments remain, and that has resulted in a dramatic expansion of the science gathering capabilities of Hubble over time.
Also, there has been a Hubble successor in the works for quite some time. The JWST is far larger and far more capable than the Hubble in many ways (although it lacks some capabilities the HST has, such as UV and full visible light coverage) but it is a hugely expensive project that is substantially over budget and behind schedule. But once it gets into space people won't be asking this question any more.
A: Money and willpower.  With any program (scientific, military, public works, etc.) it all depends on the amount of money someone is willing to put to it, and how much backing and protection that program has from getting money re-prioritized to other projects.  You are making a false dichotomy of attempting to present our past actions as a justification for actions we should have been able to take.  With the decisions made on many levels (i.e. to fight several wars, cancel various lift vehicle programs, etc.) that just doesn't translate very well.  Keep in mind that getting to the moon was all part of the "Space Race" which had many layered motivations, with science perhaps only being a side benefit to the projects.
The James Webb Telescope is the next generation telescope that is due to go up.  Although, the JWST is optimized for the infrared spectrum.  For visible spectrum telescopes, the most ambitions space based one planned is the Terrestrial Planet Finder.  However, the Hubble is still the belle of the ball.
This of course doesn't touch on the ground based observatories we have, some of which are truly spectacular!  I want to make a family vacation to Chile just to see some of them!
A: We do have larger telescopes, but they are located on the surface of Earth. The Hubble represents the largest aperture optical telescope which would fit in the shuttle's cargo bay, which was the main design limitation. We also have more modern telescopes in orbit, but they use different spectral ranges that the Hubble.
A: Given that HST already exists, it's been true for a long time that there is more scientific "bang for the buck" in building instruments whose strengths correspond to HSTs limitations, rather than trying to build an instrument that has the same strengths, but better. There are many telescopes and instruments that see different wavelengths, have wider apertures, or much larger fields of view, and so can do science that HST cannot.
Would a "Hubble but better" be a fantastic scientific instrument? Absolutely! But with the HST already there, you can probably get a better scientific return by building other, more complementary instruments. 
