Explain how please? It isn't an early enough POD to affect his birth, his personality, his raw skill and behavior. ASB I say until shown evidence, with respect.
Multiple reasons really.
My scenario calls for Joseph II living longer and continuing to antagonise the non-Germans which provokes the many ethnicities of the Hapsburg Realm to rise up and rebel (those who aren't already rebelling that is).
This means that first off Joseph II not Leopold II is leading the Hapsburg Realm during the 1790s. Joseph II was not at all close to his sister, unlike Leopold. He wouldn't have made the same offer of shelter to Marie Antoinette that Leopold did. Also with Austria in the middle of multiple wars it is in no position to provide any sort of aid to the French royalists.
With no offer of shelter and no possibility of help from Austria, Louis and Marie Antoinette are going to act more cautiously and something light the Flight to Varennes is more unlikely.
Also with Austria in turmoil you change the geopolitical situation. OTL Leopold was wary of militarily involving himself in the French Revolution, because he knew Russia's urgings to him to involve himself were just a ploy to have Austria distracted so Russia could make plays in the East. If Austria is already distracted with its own problems, Russia isn't going to make diplomatic pushes to get Prussia and Austria fighting west of the Rhine.
OTL Prussia was cautious of moving against the French Revolution until they could secure Austrian support. But if Austria is fighting it's own rebellions no Prussia can't get Austrian support and their reluctance to attack France will remain.
Britain OTL wasn't ready for a war in 1791, but got involved because Austria and Prussia were going to do the heavy lifting. Britain as always provided monetary and naval support as well as some Continental troops. Without Austria and with Prussia maintaining that 1790 reluctance, Britain has no way to take on the Feench revolution itself. In fact without Austria a lot of countries aren't in a position to fight the French Revolution. Because in the early FRW Austria was providing a lot of troops and a lot of diplomatic effort against the French Revolution. But with Austria gone there goes so much of the war effort.
So essentially without Austria you are changing the beginning of the FRW. And the beginning of the wars when France was imperilled by the Austro-Prussian invasion had a huge affect on the Revolution. It caused a feeling of insecurity and pushed France more towards radicalism. Which provoked the instability in which military officers became the stars of the Revolution and saw them rapidly rising through the ranks and saw them gaining vast quantities of political power.
In essence without Austria's involvement there are huge butterflies affecting the nature of the latter half of the French Revoluion.
And this is not even touching on the potential butterflies caused by the more successful Belgian Revolution and the Milanese uprising on the French Revolution and on Napoleon, respectively.