I will try to explain it in a simpler way for u.The first ever true computer,ENIAC was made operational on July 29, 1947.That marked the start of the era of computers.But that does not mean we still use those vacuum tube based computers.Today im writing this piece of post using my new laptop that is based on an i3 processor,that runs the latest version of Ubuntu.U will not fail to notice(i hope),that technology has evolved by leaps and bounds.
Now that was just an analogy which will help u to understand(i wish).Space technology is one of those few areas which need lots of funds,lots of research and up-to-date technology.The space agencies generally follow the Spiral model of development,which happens to be a high-risk approach.u know why??Because agencies just cant afford doing mistakes here.When mistakes happen,it results in the loss of lots of money(around 335 crore in case of GSLV D3),or loss of precious lives (like Kalpana Chawla).
Henceforth,the only way to eliminate these mistakes is using up-to-date technology.So,be rest assured,the technology that was used in 60's has evolved a lot and it is not the same.
Well,now its time for an example......
Have a look at the various Cryogenic Rocket engines used by united States.Each one comes with improvements,making the whole operation more reliable and safer.
RL10
J-2
RS-68
Space Shuttle main engine
RS-83
--n-joi!!