Most Saturday games have been a sell-out in home areas for last 2 years now tbf.
It's because they finally opened up more train routes running to South Bermondsey, New Cross Gate and London Bridge from places like Eltham, Mottingham, Erith etc etc in that time and they are areas with large pockets of Millwall support. Really made a big difference to getting to The Den for the fans that don't live in and around SE16. The floating fans that make up a lot of the 45-49,000 Wembley turn-outs.
Plus, it's the first time since the early 90s that we are a consistent top 10 second tier club. Throw in the fact that more people are taking their families, as it's cooled down a lot & lots more facilities etc on offer now.