Nobody knows why ICAO decided on hPa instead of mb especially as the value is the same.
Not strictly true. ICAO has chosen - with a small number of well defined exceptions - the International System of Units to be used as the standard system of units of measurement for all aspects of international civil aviation air and ground operations. This makes some sense - why pick/develop/use something else when the vast majority of States worldwide (although, notably, not the US) have agreed to use SI units. By practise and general agreement the units are given names related to scientists whose work was related to the element being measured; for pressure the work of Blaise Pascal was selected (perhaps interestingly, the same Pascal after whom the computer programming language is named). ICAO, since its inception, has sought to harmonise the units of measurement used in aviation - originally it recognised five different sets of units that were used globally - and adopted a harmonised set of units based on SI in 1979. In 1984 ICAO States agreed that the days of the bar were numbered and set the end of 1985 as the final date that the bar would be used in aviation. The UK held out against the change for some time and continued to use the millibar for met reports and related purposes - clearly there are a few people on this form who think this is a good thing although personally I tend to find more important things to get excited about and got used to using hPa within a day or two of their eventual mandate in UK ATC. Oh, and just in case the final bit of the puzzle is still not obvious, 'hecto' is the SI prefix for hundreds of things.