India was sanctioned in the 90's bythe US (supposedly for being communist leaning) so they couldnt get PC's which meant all they could learn on was mainframes.
In the late 80's early 90's Cobol programmers were being laid off by big companies, as it was all going PC, the COBOL millennium bug stuff was outsourced to Indians because those companies who laid off their staff couldnt get them back.
Its mainly a rumour for the UK and possibly other Western countries.
Tesco bank, TSB all use PC's, the TSB banking system was a rehashed share trading platform which is why they had problems a few years back, seems no one likes old school db transactions.
Tesco is full on in MS's pocket as is most of the NHS systems, and these are all online, when you do your online Tesco shop you connect to the store that will do the delivery because every store has a different product range and no mainframe or server could handle that for the whole country. You your GP surgery systems, regional health trusts, are all online which then gives you an idea of who is behind Wannacry considering the level of spooky surveillance of telecoms here in the UK.