This is both simple ("because inflation") and somewhat hard to answer without resorting to slightly non-trivial economic models.
Basically, if you have high expectation of inflation, you can get past a point of instability in relative simple models like Cagan's model of hyperinflation. What that essentially means is that you can't make any real "tax" income (more properly called seigniorage) that way once people expect to be taxed only that way, except by beating their expectations, i.e. devaluing the currency even faster than they expect!
And what's the problem with hyperinflation? Well, at some point shoe leather costs become overwhelming because people try to avoid holding the currency as much as possible. I.e. tax avoidance becomes currency-holding avoidance in your economy. In an open economy, where foreigners can also hold your currency, those shoe leather costs can be more mitigated (for the domestic economy) by burdening the foreigners if they have no better (currency) alternatives. E.g. one 2015 ECB working paper put the threshold at 2% for the Euro but cited another paper as finding 11% for the USD.
Besides that, there's also the point that taxes can be more explicitly targeted, e.g. you can tax "vices" like drinking alcohol, but causing hyperinflation might do who knows what to the latter. (People who despair might drink more.) Usually this is phrased as inflation being a "regressive tax" but that conclusion requires more assumptions, like for example that richer people would pay more often with credit than poor people would (for example because the rich have more collateral.)
But I'm gonna add here that this is not an either-or proposition (as the question makes it). Governments do sometimes pay a significant portion of their debts with inflation/seigniorage, in the long run, as this data on WW2 debts shows, for instance.
BTW, I should probably mention this concept/terminology distinction, since the Q does start with this assumption...
Inflation tax is the loss that is sustained by the holder of real money balances and non-indexed government bonds due to
inflation. [...A]ssuming that the
growth rate of money was equal to the growth rate of prices [...] is valid for a static economy. [... However] in growing economies
governments can increase money supply by the product of the
change in income and the income elasticity of the demand for
money to satisfy the demand for higher balances without causing
inflation. The newly issued money provides a revenue for the
government. This is seigniorage and is quite different in concept
from inflation tax.