At 7 weeks pregnant what causes the heartbeat to be low?

It may be normal. In very early pregnancy the low heart rate can be normal. Typically it should increase quickly but many times if you're doing an ultrasound very early that can actually be a normal finding. My recommendation would be to repeat the sonogram later to find out that the heart rate has increased.
Depends. If your dates have been confirmed by ultrasound, the fetal heart should be beating at >100 beats per minute (bpm). Lower rates are associated with increased risk for miscarriage (the lower the rate the higher the risk). If you are at 5-6 weeks by ultrasound, then even heartbeats at 70 BPM are within normal range and have no adverse implications.