Why are off grid solar setups only 12, 24, 48 VDC?
60VDC is the cut-off for Safety Extra Low Voltage, or SELV, as spelled out in UL 60950-1. Besides being lower voltage, SELV circuits are also isolated from the mains by reiniforced insulation, which has specific spacing and materials requirements.

In general terms, SELV voltages are ‘touch safe’, meaning that they don’t present a shock hazard with direct contact. 48V falls below this SELV threshold with some margin. It’s also conveniently four ‘12V’ lead-acid batteries connected in series (really up to about 58.8V at full float charge.)

Voltages above the SELV level are considered in the same class as line voltage, and typically require an electrician to install. Reason? Electricians are familiar with codes and techniques to protect against inadvertent contact with potentially lethal voltages, including use of proper materials, fusing, fault protection, enclosures and cable routing.

More here: https://www.edn.com/electronics-blogs/power-supply-notes/4414411/What-does-SELV-mean-for-power-supplies