It's a combination of things but mostly it's probably the fact that if a Showcase #22 will sell for those prices then a collector might think why not B+B #28?
Right now the market is valuing a SC22 1 1/2 time a BB28 in most grades. The question is is...will BB28 catch up. to SC22. In time, SC22 will be the most valuable DC SA, next BB28, and then SC4.
There's supposedly going to be a Flash movie.
If so, it could renew interest in Flash & SC #4.
The Flash needs more love...maybe a hangover in the 90's. A movie should ignite it.
SC #4 is already a pricey book, so the gains might not be as easily obtained. Remember, those $2500+ 5.0 copies of SC #22 could be had for $800 - $1000 in 2008.
SC #22 looks to have plateaued a bit now and it will be interesting to see how far it can run up closer to the movie. My guess is that the biggest gain(s) has already been captured, but time will tell. GL #1 might be a better play now (full disclosure - I am hedging with both ).
WRT B&B #28, it was only a matter of time before the big DC SA keys started to attract attention due to the run up in Marvel SA key prices. Add in the fact that WB appears to finally get that it has comic properties that can be turned into successful movie franchises IF they are handled properly (which is not a given with WB's history), and these DC books should start to close the gap sooner than later.
Besides, why waste money on a crummy 6.0/6.5 copy of a knock-off book like FF #1, whose franchise has been waning in popularity since 1969 or 1970, when you can probably land a much harder to find in grade beautiful 8.0 copy of the real deal, B&B #28, for the same price and plenty of room to run up in value?