Why did soldier get paid in land rather than gold?

As I discuss here and here and here, paying soldiers in gold requires substantial state capacity – you need a large bureaucracy that can collect taxes in cash (which requires significant record-keeping, valuation, and enforcement capacities), you need the logistical ability to mint the necessary amount of coins and ship them to the army in time for pay day, you need both the authority/legitimacy and economic development to ensure that coin can exchanged with civilians for food and other supplies, and so forth.

Thus, even in the late Roman Empire, you see this system begin to break down – the commercial economy is weakening and urban centers are declining, which means the state is having a harder time extracting the necessary amounts of gold to pay the army (especitally when the army has gotten a sense of its political power and starts demanding more and more gold), the currency is becoming less valuable as a result, which means fewer people are willing to take coins (they’re not trading as much and now you see how all of these factors are mutually-reinforcing)

So the late Roman Empire begins to shift to a proto-feudal system. First they shift to a system of direct requisition of supplies from provinces by the army (which means the army is collecting the taxes itself so you don’t need a bureaucracy to do it for them) and taxes being paid in kind (which means that you don’t need to worry about currency as much). Second, from there it’s not much of a jump to just hand over land to armies in return for military service – whether you’re talking about the limitanei under Diocletian and Constantine or the stratiotika ktemata of the Byzantines, etc.

And in the West, once the Roman Empire falls completely, it was similarly an easy shift for ring-giving kings to start giving out land, now that the Roman bureaucracy and economy that let them get their hands on gold to turn into rings to hand out went away.