For all it’s constraints Southend is in a very different place to Manston. People might critique Southend’s location and accessibility but it is within an hour’s journey by road or rail from a sizeable area of London’s administrative boundary. 45 mins to Stratford on a relatively inexpensive and regular train is a somewhat better proposition than 1h20 on a less frequent and quite expensive HS1 service. Southend’s immediate local catchment is wealthier than Manston’s. Therefore because Southend has attracted EZY, FR and others does not mean Manston can replicate that feat.
To meet the forecast pax figure Manston would require 6 daily Ryanair flights - that’s 2-3 based units. If Manston attracted Ryanair to do 3 weekly to Alicante and 2 weekly to Faro and Tenerife (for example) it would be doing incredibly well.