For airports with establsihed 'full service' operators, following the the low cost route is something of a gamble; and the bigger the size of the established operation, the bigger the gamble it is.
Low cost operators will demand reduced landing & handling fees as well as a contribution to marketing & promotion costs in return for setting up a large scale operation. Unsurprisingly, existing airlines are likely to be cheesed off if they are not receiving similar deals. More so if the low cost airline is going to compete directly with them on particular routes.
At airports like LPL, PIK etc, the decision was simple. More recently however airports like BRS and EMA have invested in attracting a low-cost operator - suggesting that they believe there is limited future growth from 'full service' airlines or that they can accommodate the two types of carrier simultaneously.
At bigger airports such as MAN / BHX, the size and range of the existing full service network means it would take a sustained and fairly dramatic reduciton in traffic from establshed operators for the low cost option to become attractive to the airport authorities.
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