I think you have to blame the Victorians for not building more of the Highland Line double tracked in the first place
Well they did build it after all!
However they were building for relatively non-intensive and slow-speed use through very difficult terrain in some of the most lightly populated parts of the country, by a company (the Highland Railway) with fairly limited financial means - so I think they can be forgiven for engineering it in this way.
The passing loops and single track sections were perfectly acceptable for the days of steam where line speeds weren't high and timetables were generous. The difficulty comes when you try and run a modern, high(ish) speed, high(ish) frequency intercity service along these same lines.
There used to be many more passing loops as the stations were generally built as 2 tracks allowing trains to cross at each station, but many of the lightly served rural stations have closed and with them the loops. BR lifted/shortened quite a few of the remaining loops in the 1960s to try and cut maintenance costs, but pretty sharpish had to reinstate them when pipe traffic for the north picked up in the 1970s north sea oil boom.
Incidentally, the line never even originally ran to Inverness at all - it ran up the Spey Valley from Aviemore to Forres, via Grantown and Dava (part of the route is being slowly re-laid as the Strathspey Railway). That was sensible at the time as that is where the traffic was in the whisky glen (coal and casks in, whisky out). It was only in the 1890s that the inconveinience for passengers heading north to Inverness and beyond was remedied by building a direct route from Aviemore, which is the current route.