No one really knows why. This web site quotes an Essex Countryside article which states:
How the Crowstone(s) became so called is not accurately known. The name may have arisen from a nearby tenement exiting in 1536 – Crowes. The again it has been suggested that at one time beachcombing crows, which colonized agricultural settlements of the district, favoured the stone as a look-out. That these numerous birds accounted in the first instance fro the naming of both the tenement and stone is sufficiently realistic to be accepted. Beyond this, other reasons advanced have no local connection.
It's location is also sometimes described as Leigh (just upriver), Chalkwell (the nearest railway station) or Southend-on-sea (the general area). I use the last of these as there are two stones in the general area so it covers both of them.
The Essex Countryside article also gives more information about the process and payment amount:
Formal rights over restricted stretches of the Thames east and west of London Bridge were first granted to city conservators in 1197, for the payment of 1,500 marks, by Richard I. Richard’s document was confirmed by King John in 1199, by Henry III in 1227 and by Edward I, whose charter of 1285 extended administration of the river, the easterly limit being determined at the Essex shore
That explains the date of 1285 on the older of the two stones (as described in the next post): the one in the photo above is a more recent, Victorian era addition, shaped a little like a mini Cleopatra's Needle.
This part of the Thames has large tidal variations, so the Victorian stone, which are a decent walk to the sea at low water (above), is completely cut off at high:
longest in the world:
It's definitely more interesting to visit at low water where there are some intriguing and I suspect historic tracks out across the mud to the distant water: