It's because of pathing over the flat junction in the station throat. Pendos 'turn left' when departing Glasgow Central, so they're platformed in the left-most part of the station (from a southbound perspective) so as not to cut in front of any other services.
Although they have been known to depart from platforms 9-11 as well.
To call platform 1 "prestigious" is going a bit far. Certainly, platform 1 is often used at smaller stations for services in the "main" direction of travel - for example in the Greater Glasgow area, Platform 1 is almost always used by trains traveling to the city and Platform 2 for trains traveling from the city.
There are instances where Platform 1 is generally used for the longer-distance, more "prestigious" services, like in Glasgow Central with the Pendolinos using platforms 1 and 2. However there are also many examples where Platform 1 is far from prestigious. Glasgow Queen Street's P1 is a short bay, largely used by Anniesland shuttles. Bristol Temple Meads' P1 is also a short bay, that
from what I've seen is in the dingiest part of the station and could hardly be considered prestigious. Aberdeen's Platform 1 is currently a dilapidated siding that's not in passenger use. Stirling's is a car park. Haymarket's Platform 1 is mostly used by arriving passengers. Edinburgh Waverley's is usually used by local services (and is not very busy). I could go on...
I'd like to question use of the word "prestigious" though. To me it implies importance, and Virgin's London-Glasgow trains are a vitally important transport link, but I'm sure that a passenger traveling on the Cathcart Circle will see their little 314 as being just as important to them.
The much less glamorous reality of platform numbering is that platforms are generally numbered sequentially (or at least there is or has been some method to the numbering, even if it's not immediately obvious). Trains are put onto a platform that they will fit into, and that is the most sensible for the signallers, looking at their journey through the station and the potential for conflicting movements (I'm sure I'm oversimplifying this - if any rail staff want to correct this please feel free!). As has been said, the reason the Pendolinos in Glasgow use P1-2 is that the platforms are long enough to hold them, and provide a route to the WCML with minimal conflicting movements. Similarly, in Inverness, the Chieftain uses Platform 1 largely due to the platform length above any other concerns.
The "prestige" of having a certain service use Platform 1 will generally not come into the equation.
