I think you have made that up. The HSTs had been covering the intermediate stops since 1983 so hardly a case of “aren’t suited”. The reality was MML were undertaking a big timetable change/uplift and there were no spare HSTs available.
Absolutely not!
I commuted into St Pancras from Wellingborough late 1980s to 2000. In Summers, the (then Paxman-engined) HSTs ran on One Power Car Only so often that commuters used the acronym 'OPCO' in conversation. Occasionally, the number of power cars serviceable dropped to a point where the timetable was abandoned for a few days while Neville Hill got some working again. I personally have arrived at St Pancras at around 1730 and found the station empty of trains and the next inward service (not 'cancelled') a booked arrival of 1800 and running late. I have also been on trains that have ground to a halt after the leading power car has emitted smoke so dense that visibility outside is nil; when the entire contents of the engine sump have been ejected skywards; and ,while waiting time, dense smoke and then flames from the exhaust. Entertaining days...
During those times, MML management on posters used the reason I have stated (amongst others) to explain unreliability and verbally by staff. It was also the case that articles in Modern Railways about the introduction of. Cl 170 ("Turbostars') gave this as the reason or HSTs not calling South of Leicester.
More recently, it was given as the reason for the HSTs being confined to the fast Nottinghams in their dotage, with the resultant poor diagrammed utilisation.
Cancellations because of poor availability of trains (Cl 360, as well as Cl 222 and Cl 810) is happening at the moment hence my remark 'the more things change the more they stay the same'. But the reason is always outwith MML/EMT/EMR managements' control, apparently.