It's quite interesting looking at the pricing in places like Leyton. While it has cool stuff moving in, it's still mostly grubby and quite deprived.
The pricing looks similar to Bromley which is NOT deprived and grubby, with far far superior schools and a nice town centre (Waitrose, Carluccio's, Massimo Dutti etc mostly chains so not cool).
Because it is on the central line.
Because it is popular with priced out hackney people who value being bikable to cool shizzle like hackney wick, Victoria park, london fields, broadway market, columbie road flower market etc
Because you can get to canary wharf or your city desk in 30 mins either by bike, or by tube
Because it is much cheaper in a taxi to get home to Leyton than to Bromley after a night out, or indeed, you just hop on the 24h central line.
It is obviously pretty deprived in places, that is what makes it both well connected and also affordable.
Its no Sutton, which you obviously favor ;-)