maximum achievable bandwidth for that opamp ft(max)/10
Could anybody explain why maximum achievable bandwidth for that opamp
  is ft(max)/10 instead of ft(max)?


Here's my take on it....

It alludes to the axiom that for a given device to produce a "specified" voltage gain, you need the current gain of that device to be at least ten times that specified voltage gain value. In following this "rule", you end up with the final circuit being fairly stable and not too prone to temperature effects (which might reduce current gain) for instance.

So, if fT is 300 MHz, you could make a fairly reliable voltage gain circuit that has unity gain at fT/10