The ticket sales merely show the tickets that people buy. They do not show the journeys that people make, which can often be different.
Many people are under the false impression you 'have' to pay separately for the Underground or any 'local' train journey at the London end. Or they may simply decide to use PAYG for that section because it's cheaper.
So I wouldn't be reading very much at all into Kings Cross being the official top destination. Clearly most people are continuing onwards using the Underground, bus or alternative trains; whilst the Kings Cross area is now more of a destination than it used to be, it still doesn't have the draw that the West End or City does.
The reality is that using PAYG for your local London bit is usually cheaper than buying a through ticket, especially if the tube is involved. For example, if I pick a few places in all directions around Central London within the Oyster area and compare the same Advance price level (the below use ticket code BUS as an example):
NCL - KGX (Kings Cross): £84.50
NCL - Underground Zone 1: £87.50
NCL - IFD (Ilford): £91.30
NCL - HKC (Hackney Central): £87.10
NCL - AAP (Alexandra Palace): £87.10
NCL - WHP (West Hampstead Thameslink): £84.50
NCL - EAL (Ealing Broadway): £91.30
NCL - WIM (Wimbledon): £91.30
NCL - ECR (East Croydon): £91.30
NCL - LEW (Lewisham): £91.30
with the off-peak Oyster / contactless fare from Kings Cross / St Pancras (NR or LU as appropriate to the journey) as follows:
to Underground Zone 1: £2.70
to IFD: £3.20
to HKC: £2.80
to AAP: £3.50
to WHP: £2.80
to WIM: £3.00
to EAL: £3.00
to ECR: £4.20
to LEW: £3.10
Among the above, most of the places I listed are cheaper by paying separately using Oyster or contactless for the local leg, apart from the stations close to Kings Cross.