Just looking at some features of this service around 1955 gives good reasons why patronage had fallen away.
The new Mullard factory was built on a large out-of-town site, not far from the station, which had also in Victorian ways been built nowhere near the settlement of Simonstone itself, being in open country.
The road to the station had (and still has in parts) no pavements, so for those without cars from Simonstone village would be pretty useless - and of course for those with cars, now likewise. The no pavements sections include the walk from the station to the factory entrance.
The factory entrance is built on the south side of the big site, opposite side to, and a considerable walk from, the station. It's obviously been built to face the main Blackburn to Burnley road.
The main road has a bus service every 20 minutes or so from Blackburn and from Burnley, stopping on the main road at the factory gate. It would also stop all the way out from Blackburn where more convenient to passengers' houses, instead of just the central station. I read an account of just this route a while back from someone who long ago went to school on it. Apparently in the morning as many vehicles as required were provided, so although timetabled still every 20 minutes, 3 or 4 vehicles might be operating a peak time run. That's the sort of flexibility rail operators seemed unable to provide.
Morning trains from Blackburn stop at Simonstone at 0551, 0802, and then 1228. The bus is every 20 minutes. The train under discussion, the 0632 from Blackburn, is actually nonstop at Simonstone.
Road from station to factory today. Even now, no pavement :
Simonstone Ln - Google Maps
Train timetable 1955 :
Bus timetable 1965, including specific "Mullard factory" stop