Because two-century old injustice is handwavey pie-in-the-sky talk,it is also ridiculously selective. There are other injustices too and how do you determine what the amount is? Do you offset slave ancestors privilege for growing up in the 1st world against their ancestors losses, what is the actual tangible loss due to the past rather than those due to more modern injustices such as overt racism or mere growing up in a poorly run town? It's completely impractical and/or virtue signalling.

Reneging on debt is a very real decision taken today (not in the past). Practically speaking it signals to bondholders you can't be trusted to keep your word. Hardly honourable, not that they'd worry about this point of view, it's cold hard calculation, will you renege when you proceed on your next ideological crusade, this signals yes.