Why do I only have 30V going to my light fixture
Are you sure the circuit was live when you measured it? If you try to measure an open (i.e. off) circuit, you can sometimes read a "phantom voltage" caused by coupling with other nearby live wires. Phantom voltages can't generate a significant amount of current and are therefore harmless, but they can be measured by a multimeter.

If you say the fixture doesn't work even though the old one did, perhaps a connection came loose somewhere and now you have an open circuit with only the phantom voltage. Or maybe you just forgot to switch the breaker back on?

I don't see how the old fixture could've worked if it was actually only getting 30 volts, so something must've changed during your installation.