You already named the weight difference.
Other differences are: cost reduction, competition of other brands, kerf of the blade (2.2 mm) and use of FSK rails attached to the saw.
Most battery powered tools now are 18V, so Festool also wanted to have a circular saw with 18V.
BTW. Cutting capacity is the same 55/50 on rails. But the rpm of the HKC is somewhat lower (4,500) compared to the TSC (5,200).
When you use 2 batteries and use the right saw blades you nearly never will run out of power.
And the TSC will also run on 1 battery.
Even Mafell has just released an 18V cicrcular saw. In the past they only had huge 24V and 36V circular saws.