Apparently they can't fill the seats against proper revenue to SJO.
A not entirely unexpected 'tongue in cheek' LH Group response
Considering that KLM will increase frequency from 3x weekly AMS-SJO-AMS to 4x weekly AMS-SJO-LIR-AMS, your point that KLM can't fill seats is unlikely to be the case.
They are adding capacity and opening a new market.
LIR is a Winter sun hot spot for North Americans with service from a.o.:
- Air Canada Rouge - YYZ
- WestJet - YYZ, YYC
- Eastern Airlines - YYZ
- Sunwing - YYZ, YHM, YUL
- American - MIA, DFW
- Delta - ATL
- United IAH & EWR
- Jet Blue - JFK
- Southwest - IAH
- Alaska - LAX
- SunCountry - MSP
So KLM adding the second European LIR connection (after TUI - LGW) is in line with its relatively strong history in Central America.
Offering four weekly service from the start, shows faith in both the commercial potential, while providing operational improvements at the same time (No unscheduled CUR stop required).
Question is will STAR Alliance offer service from Europe?