Flirts might not yet be as reliable as HSTs but they've only been in service less than 6 months.
How long have Scotrail had HSTs for...? and we are talking about them being quite reliable since the new year ie not quite 2 weeks!
HSTs have been reliable, seemingly it's been crew knowledge that was the issue (rumours abound of fitters not knowing how to reset APT speedometers etc), as 40+ year old stock doesn't just become more reliable over the christmas holidays, although it does appear the first few sets delivered by Wabtec were rather questionable, as from the 7th set onwards they've all entered service within a few days compared to potentially weeks with early delivered sets such as HA19.
But had they gone with Stadler, then they would have had no flirts in service for a lot of months, compared to a handful and of classic and refurbished HST sets, not to mention they would probably be given lower priority than GA, we could be talking only the first sets in driver training now, compared to the progress of the HST programme. IIRC there is doubt the diesel gen sets on those will fit through a few tunnels up north anyway.
IIRC FirstGroup or one of the other bidders proposed new build 68+Mk5a sets instead of HSTs.
My understanding was all the other bidders just said something along the lines of displaced stock from the EGIP would go to work for the services HSTs are now serving, which is why Abellio's proposal was chosen as it promised a vast improvement instead of just doubling up 158s and 170s.