The E175's are a great option for Flybe but unfortunately they chose to cut costs and have de-rated engines on them (The main reason their performance is so poor from SOU) whereas the E195's do not, meaning they have a higher thrust-weight ratio + also a larger wing area which means they are far less restricted from SOU (Lord Spandex Masher's post details the limits of the E195 from SOU very well).
Its a real shame as if the E175's were de-rated I could have easily seen quite a few of the promised routes and others also being added and working well and also making Flybe's 'mini hub' at SOU a more attractive option to those who use LHR/LGW for their short haul travels.