Combination
I think there are a combination of factors here.
Price - RE are expensive when compared to FR/EI/BD on London. So pop 2 people in a car and you are in DUB or ORK. Southend may end up working for RE - ie it's quieter and quicker to get out of. But the message isn't dropping yet that SEN is handy to get out of etc. The business guys will twig it quickly enough.
Regards
Shamrogue