By saying it is only 'at Mernda' makes it seem as if it is isolated to one individual suburb, not an issue across the city. All lines to the north and west of the city have abysmal service on Sunday mornings, for example. People travelling from the west and south-east and south will most likely use routes that include buses which also have very poor service.So the issue is the public transport at Mernda, not the public transport getting to the beach.
The problem is the public transport network as a whole is not well equipped for many kinds of trips. Sure, Sunday timetables are just something ridiculous in and of themselves; but weekend services overall are quite poor (especially on the bus network).
Beach trips also tend to involve non-radial trips (eastern suburbs to the south for example), tend to occur on weekends on public holidays - when public transport services are lower and congestion is also lower; and often involve multiple people travelling together and carrying a bunch of stuff, which makes driving advantageous.
So there is poor service, not as much disincentive to drive (parking can be a pain, but not as bad as a peak hour CBD trip), some natural advantages to driving. I can easily see why driving to the beach is much more attractive.