While there's nothing to say that a mortgage wouldn't be a workable financing solution for a property bought at auction, mortgages aren't especially well-suited for this type of transaction, which moves very quickly.
While it's not unheard of that mainstream mortgage lenders can issue a mortgage very fast, it's the exception, not the norm. Lenders - especially if you require significant capital - generally won't be able to meet the 28-day deadline you need to complete the purchase after the auction. If you can't make the cut-off date, you'll lose both your deposit and the property. Generally, trying to organise a standard mortgage in such a short period would be highly stressful, and you'll constantly be worrying about receiving the funds by the completion date.
It's also not unusual for auction property to be in a poor state. For buyers, that's often not an issue. You might be interested in the property precisely because of its excellent development potential, or you're planning on undertaking significant renovations, for example. Lenders may well be less keen to grant a mortgage in such situations, however, and lenders will only give mortgages on properties they deem to be 'mortgageable'. To be considered mortgageable, a property will need to meet certain requirements. A property can be unmortgageable for any number of reasons. If the property isn't weatherproof, isn't habitable (when basics like the kitchen or bathrooms don't work), or if it has anything like severe damp issues, it's unlikely to be mortgageable. In such cases, you'll need to find an alternative financing solution.
All in all, mortgages aren't always practical for buying at auctions. Auction finance is a tailored solution designed to meet the different demands of auction purchases, and it is likely to be a better fit than a traditional mortgage.