I would suggest it comes down to staffing costs / budget. As soon as you run both stations on Oblivion you need two more staff on the platform - so an extra £16.42 per hour to run the second station.
Staff costs are way the biggest operating cost for any park. Cutting back on staff here and there all adds up over the season. Back in the early 2000's the big coasters all had staff on the ride entrances to enforce height restrictions / assist guests. All the big rides generally had a bay-loader in the station too. In most cases both have gone & height restrictions are enforced in the station (slowing down throughput) and guests are often left to bayload themselves (reducing throughput).
When Virtual Queue [the precursor to FastTrack] was first introduced on Nemesis it required six members of staff to operate (as Towers were still experimenting with it). Six became five pretty fast, now it's down to 1 poor person on the merge point and that's it. Ticket sales are all taken care of in the Box Office that is always understaffed too - some of the queues in there can be silly (& this is when Towers are trying to up-sell & extract more money off you!)
