Why were germanium diodes so fast and germanium transistors so slow?
Major factors affecting bipolar transistor frequency response include:- 


minority carrier transit time
junction resistance 
junction capacitance 


These can all be reduced by reducing the physical dimensions of the transistor. However in the early days this was not so easy.

The first transistors used point-contact junctions, which themselves had small dimensions (which is why point-contact diodes work well at high frequencies). However carrier transit time between Collector and Emitter was proportional to the cube of their separation, so performance diminished rapidly as distance increased. To get a cutoff frequency of 20MHz required that the contact points be only 1 mil (0.025mm) apart, which was difficult to achieve and maintain. 

Inside 2N110 Point Contact Transistor:-



Diffused alloy junction transistors were more robust and easier to manufacture, but had larger junctions that increased capacitance, and risked shorting out if the diffusions got too close together. Again, high frequency performance could be improved by reducing dimensions, but there was still a limit to how small the structure could be made.

The photo below shows the insides of two popular Germanium transistors. In the AC128 (an audio frequency transistor rated at 1A) we can clearly see the rather large Emitter and Collector pellets on either side of the Base. On the AF106 (an RF transistor used in TV tuners) the semiconductor part (hidden under a protective coating) is much smaller. This transistor uses 'mesa' construction to reduce the active area. It can only handle 10mA, but maximum oscillation frequency is 1.2GHz.