I think this is to do with the virus running it’s course. They need lockdown (or current restrictions) to enable the health system not get overwhelmed.
Part two will be a general release of restrictions but as the virus runs its way onto those unaffected the potential for hot spots still arise. The major hubs in terms of population are key. If you can control those throughout that is good.
6 Months and I’m assuming figures have been run on infection rates and modelled against the health system and its ability to cope.
Thus as initial spikes subside the restrictions ease and if potential for another spike becomes a reality then restrictions are applied again.
The UK is also staring six months to ease this through.