that older question re-iterates what I already know... statehood is "granted" arbitrarily, probably by combination of military power and emotional factors
So I guess you're looking for the specific factors that distinguish Catalonia from Ukraine? Here's a few:
- Ukraine was made a UN member by the initial deal with the USSR (which basically got 3 seats). That made it hard[er] for anyone later to argue Ukraine is not a country, although Russia tried that more recently, more or less formally.
- Russian leadership itself was interested in breaking the authority of the Soviet leadership, so hard for them to argue at the time at the time of the breakup that other Soviet Republics were not entitled to that.
Similar "emotional" factors applied to other breakups like Yugoslavia (somewhat, the Serbians were not excited, but most other former members were) or Czechoslovakia. If several large-ish Spanish regions (not just Catalonia) decided they want Spain gone and managed to somewhat coordinate their efforts in that regard, who knows what might happen.
OTOH the international community (except for the Taliban-ruled Afghanistan) didn't recognize Chechnya's [self-proclaimed] independence. (It looks like the Chechens did it several times in the 1990s. The most democratically credible attempt being perhaps in 1997, when Russia+OSCE organized elections, but the resulting Chechen parliament declared independence [again]. After the Russians and the Chechen factions allied with them [i.e. Kadyrov] got the military upper hand, they organized a counter-referendum in 2003, reaffirming Chechnya's membership in the Russian Federation.)