Possibly because in the US there are so many more prisoners held on minor offences (drug possession, etc), who wouldn't even come close to considering suicide?
Per million population, the numbers become 1.43 in E&W and 1.63 in the US.
(NB, just England and Wales. Justice is controlled by the Scottish Parliament in Scotland, under a different legal system, so the figures are separate: 13 suicides (~0.16%!), 4.19 per M population. Small country bias, or is Scotland a particularly grim place to be imprisoned?
Northern Ireland: no suicides since 2010 [5 in the last decade], prison population 1465, country population 1.8M).