Different reasons. Some local lines were hugely unprofitable, served tiny communities and still would (e.g. Bluebell south of Horsted Keynes), some were shut for housing / road building (Lewes to Uckfield, Horsham to Shoreham), some because the primary traffic dried up (mostly agriculture going...
For those of you who haven't recently been to Eridge station, this mural depicts the disused railways in the box formed by the BML, Hastings line, East Coastway and Redhill - Tonbridge lines