Just for the record, there have been several versions of the CF-34, with differing numbers of discs in each compressor or turbine stage. Current CF34-10 has:
fan - 3 LP compressor discs - 9 HP compressor discs - 1 HP turbine - 4 LP turbines.
However, your question still applies.
Basically, you are seeing the Law of Diminishing Returns in action.
As air is compressed coming into the combustion chamber, each compressor disc is having to add more and more compression to already compressed air. So it takes a lot of stages to produce the last few percent of "squeeze" before combustion.
Once the fuel/air burns, the hot flow out of the combustion chamber is at its peak pressure, so it takes only one (or two) HP turbines to extract enough power to drive the HP compressor.
By the time the air gets to the LP turbines and the pressure is dropping, it takes 4 stages to extract every last drop of power from that low(er)-pressure exhaust.
Which is the important power from the point of view of flight thrust, since it is what drives the fan, which is producing 75-85% of the total thrust (5:1 bypass ratio).
In other words, the LP turbines have to do most of the work, with less pressure available. So more turbines.
The HP turbine has less work to do (it is essentially just a "gas generator", to use the turboprop term) with more pressure available, so you only need 1 or 2 discs.