The reason has something to do with the nature of the cpu making process. Just to let you know, there are some variants of desktop cpu even though they have same architecture, take i7 for example, why do you think one sells more expensive than the other? The reason being the most expensive one (9 series) has least defect in the cpu, enable it to utilize the most transistors and this is pretty rare, therefore they priced higher.
All cpu were made in the same process, the one that have more defects will priced lower, they in fact shut down the defective part and mark them 'more energy efficient' while they are effectively running less transistor.
The same thing happened here, not all cpu is perfect, so not all cpu can be clocked high. So they decided to take 1.2ghz, as it is probably where most manufactured cpu able to cope with. i.e. if you device can run 2ghz or more that mean you get the almost perfect cpu, as simple as that.
