When you have a weekend you can and usually do go anywhere within range. Hmmm, raining on Slesse, let's climb at Squamish instead.
But when you fly in from distant lands with a week off from work and Rainier the next tick on your road to Everest and the 7 Summits, you may not have brought enough stuff or have the mental attitude to pack it in on Big R and head to Vantage instead just because the forecast is crappy for 3 days down the road.