I can see why 'indoor boot sales or 'table top sales' might largely have replaced jumble sales:
A jumble sale requires the organisers to spend weeks persuading people to give donations, which they've then got to find space to store them in. Then they've got to sort them and price them. on the day of the sale they'll need quite a few volunteers to man the stalls. Once the sale is over they'll be left with loads of stuff that they've either got to store until a future sale or find a way of disposing of.
A table top sale requires nothing more than putting up a few notices and taking bookings over the phone. (i.e. one person can do it on their own). On the day of the sale it's possible for just one person to put up the tables and to take the money from sellers (as well as from potential buyers if an admission fee is charged). At the end of the sale sellers will take all of their unsold stuff home with them.
So a jumble sale involves quite a large number of people to work hard over quite a long period (and for people to donate surplus goods, rather than to have the chance of making some cash from them), whereas a table-top sale can be arranged and run by just one person with only a small amount of effort. ( I recognise that most such events actually have a small team running them, often to help with catering, etc but it's still far easier to organise and run a table top sale than a jumble sale).
However we do still have a few jumble sales around here. Cats Protection run then occasionally in Stowmarket and other organisations sometimes host them in church or community halls in local villages.