Did Surrey get lucky or is it just that counties like Sussex and Essex are much more rural to have had more cuts?
Surrey has 84 stations; Hertfordshire has 50 (47 if you don't count Moor Park, Croxley and Watford Met) with a near-identical population and area. Around fifteen of Hertfordshire's serve the six towns with over 50,000 people (Welwyn and Cheshunt were over 45,000 in 2011); Woking, Guildford, Walton, Ewell and Esher have around ten. So Hertfordshire outside the largest towns has 35 stations for 650k people; rural Surrey has 74 for 800k.
(These calculations should come with the caveats that a) some smaller towns might have multiple stations, b) more stations doesn't necessarily mean more lines, or more useful services, and c) it might be that what's considered as London's urban area includes more stations in Surrey than in Hertfordshire.)
I'd say that part of the difference might be that Surrey's population's a bit more spread out than a lot of other southeastern counties - no Medway, Reading, Southend, Brighton, Luton, Southampton-Portsmouth - and maybe partly that the North Downs line was better-planned or just luckier in terms of which towns (/ airports) grew than other lines.
In terms of Essex being rural, I think Essex fared quite a bit worse than Kent - no way of getting between two of the five busiest stations in the East of England within the same county by rail without changing twice or going via London - whilst as has been mentioned travel within Kent generally seems to be quite good, or at least good enough that it isn't generally necessary to leave the county to go between towns within Kent.