Re Hearts I pointed this out a few years back. Their manager got poached by and he walked out for....MK Dons.
It's not the clubs' fault, the Scottish Leagues get buttons in terms of TV revenues. The Old Firm matches must get sold around the anglophone world so the Scots and esp Irish diasporas get their fix, but I reckon that most watch on streams.
It's hard now to recall how big Scottish football was at one stage. The late 60's for instance:
1967 - Celtic win the European Cup . Incredible, all the players bar one grew up around the corner from Parkhead. The exception grew up 30 miles away;
1967 - Scotland beat WC winners England 3-2 at Wembley;
1970 - Celtic knock Leeds out of Euro Cup semi. The crowd? 137,000. Had to hold it at Hampden.
1937 - Scotland v England at Hampden, 149,000.
1937 - Scottish cup Final Celtic v Aberdeen, 147,000
1939 - Rangers v Celtic, 118,000, record League crowd
Not just professional football.
The Junior Cup? 1951 - 77,000 went to Petershill v Irvine Meadow
In the late 1980's the game had an artificial boost when English clubs were banned from Europe, Rangers and Celtic bought players from English clubs. Temporary of course.
TV has created such a gap now between the English and Scottish leagues. The EPL has been a phenomenal financial success. SPL only has Rangers and Celtic, a poor man's Spanish League. The Old Firm's only chance would be to join the English leagues and wait till they could get promotion to the EPL, but it won't happen. If they could, they could eventually compete for top 4 and titles, the size of their respective fan bases is enormous.
Final note on those massive crowds in the cash-only era. Clubs would routinely under-report the size of crowds so as to evade tax.