The minimum start frequency is set to 10 Hz because of the extended acquisition time required to collect enough data to ensure a useful level of measurement accuracy. If the low limit was 1 Hz, the scope would need to test 10 cycles (to maintain specified accuracy), so the time base will be 10 s/div. This would make for a very slow test.
Other issues: For low frequencies/long acquisitions, the cabling/etc can be easily disturbed, causing the scope to re-acquire. So we limit it to 10 Hz.