I cant remember the central part of the country was in plan for 2nd airport under any government. Mattala was originally planned some where in Monaragala under previous UNP regime.
There could be some technical reasons for this.
1) In an event of bad weather at KAT , you have to divert the traffic to somewhere else.
2) At the moment all flights coming to Sri Lanka bring additional fuel to fly Chennai as there is no facility to land anywhere else in Sri Lanka.
3) This is one of the main reasons for going for a second airport.
4) Note: With the opening of MRI (VCRI) they can bring more payload thus reduce unit costs.
5) Puttalam, Kurunegala and Kandy areas located West to the mid country mountainous range, get same monsoon winds and cloud cover with Katunayaka and we know the entire area get heavy downfall in SW rainy days.
6) As such, in a severe weather situation another airport located in the same weather region is useless.
7) Going towards Kandy, you cannot find flat lands and enough open area to design a clear approach, thus cost will be very much higher if we construct a big airport with 11,000 ft runway.
8 ) Minneriya / Hingurakgoda was in the list sometimes ago. People were on alert for losing their developed lands and buildings. Recently there was a protest for extending Sigiriya runway by archeologists and general public. As 'seek' said Moneragala was a very good location in the list.
9) Actually the original work started to build this new airport in Weerawila extending the present runway and acquiring thousands of acres of rich farm lands of right bank of Lunugamwehera scheme. Strong protests aroused because the paddy lands, roads and buildings of the area were already developed using billions of foreign loans. The site was then shifted to Mattala due to this protest . ( Some technical reasons also came in to light.- bird migration hazards etc. )
10) With or without all the above reasons, I think South got precedence.