I'm sure as an established operator, they will have done their sums and worked out what is best for their business. Most passengers are not going to be paying £3.50 for a single ticket, because they offer all-day tickets for £5 and further discounts for weekly tickets or group travel, making them cheaper to use than most operators with capped fares if you're making even a simple return journey.
The £5 was a reduction as a response to Stagecoach competition gutting passenger numbers on the routes that they have extended over.
First, Preston Bus extended their 23 over part of the Stagecoach 125 route, as far as Bamber Bridge.
Stagecoach then extended their 111 over part of the 23 and 35 routes, including a section of road that doesn't have a direct bus route into town (apart from the circular 48 that takes the best part of an hour to reach the city centre via ASDA and Longsands).
Preston Bus panicked about Stagecoach taking them on seriously and introduced the £4.20 return on the 35, in addition to the existing £4.20 return on the 23/19 Preston city centre - hospital combo.
IMHO Rotala miscalculated here and pushed locals too far with £3.50 singles, even with a day ticket Stagecoach (or even an Uber in the non-Stagecoach areas) can be cheaper than most Preston bus tickets bar seasons, so they lose a lot of discretionary travel.
Their network is primarily city-based too and doesn't have the reach outside into suburban areas e.g. Transdev or Stagecoach do.
A Stagecoach return is often a bit cheaper than the £5 day ticket if you want to make a comparison with singles (closest day ticket equivalent with Stagecoach is the £5.20 Preston Citi DayRider) - for example, it's £2.20 single from Preston station to Lytham Road on the upper Plungington/Fulwood border.
80p cheaper than Preston Bus, and for only £1, you get a ticket that covers the suburbs as far south as Longton.
For £6.50, you get a Central Lancs day ticket that offers you travel the entire way to Bolton.
Preston Bus just don't have that reach or provide such good value.