The reason why people were using the buses less was down to them cutting the routes. People found they had to walk further to stops, plus the other reason, if the only place you could go was the town centre, then why bother.
Don't forget a few years back, they had major roadworks down Paddington/Penzance Drive which caused little problem to the general traffic but caused major delays to the bus services, which was probably another reason why people gave up using them. Even today I see major traffic jams down there, usually down to the fact that the exit onto Meads Roundabout is the only one with no traffic lights.
I've even started using taxi firms over the last few years to get around Swindon as the buses take far too long