Multi-stage differential amplifier
Like with opamps you have a choice of gain versus bandwidth. In the first stage the gain is limited (compared to a solution where there would be a PNP current mirror instead of R1 and R2) but you get more bandwith in return. If you need more gain than this first stage can privode, just add a second stage which is what has been done here. You would't want to maximise the gain of the second stage (and thus limit the bandwidth) so again a resistive load is used instead of a current mirror. Obviously the designer wanted a single-ended output so that is why only Q5 has a load resistor. Giving Q4 a load resistor as well doesn't change much to the overall design.

So: less gain but more bandwith !