You do have to ask the question if he’s so good why Haven’t one of the 2 big clubs snapped him up ?
I think there's a culmination of reasons. He's only ever performed at Scottish Championship level which is probably comparable to a mix of League Two/National League. Given his lack of experience at a reputable level the price tag on his head is likely to be completely inflated - especially considering reports north of the border suggest that his goalscoring record actually masks a complete lack of footballing ability. Also, if you look at the quality of players Celtic and Rangers have brought in, in recent years, it's either the cream of the crop from the SPL, they've dipped into the European market for exciting talent, or occasionally manage to attract young prospects from the EFL/Premier League. They're fishing in far bigger ponds than the Scottish Championship and are likely only to invest such large sums in players lower down in Scotland's footballing pyramid if they're a real prospect. Shankland is 25, 26 in August. Hardly much room for development at his age.
Also, with regards to EFL clubs choosing not to take a punt on him, it's likely to be similar to why Celtic and Rangers haven't. Except I think you can add that there's now a stigma attached to Scottish strikers after so many have failed to adapt to the English game following 'big money' transfers.