A couple of bits of history.
a. Under the previous (Lab/Lib) Scottish administration the local Regional Transport Partnership (Sestran) set up a major project to upgrade routes between Edinburgh and surrounding areas (to include A8 and A90) including a £4.6m budget over a number of years [Spokes Bulletin 94]. This was something Spokes had lobbied really hard to get, so it was fantastic. Sadly, when the SNP came to power they removed all capital funding from RTPs and gave it instead to councils, to spend on whatever they liked - not even necessarily transport [Spokes 99]. By that time only the first year of the new project had happened - this included the nice new A8 section on the north side and various main-road cycle upgrades in Midlothian elsewhere, but then all the plans ground to a halt.
Because these routes tend to be away from the centres of population they have lower potential usership than the inner urban routes, so the local councils (such as Edinburgh, West Lothian, etc) tend to concentrate their own cycling efforts into the built-up areas - you can see their point to maximise usage per £ invested. The RTPs on the other hand had a regional perspective (Sestran covers Fife to Borders) so could concentrate on routes like A8 and A90. Now there's no obvious source of money to implement regional infrastructural projects like this one - it's a public transport problem too.
b. Re the muddy field to RBS site, when RBS was seeking planning permission there were forceful submissions to the council from spokes and CTC calling for decent access routes (including that one, which I seem to recall is a right of way). The cost would have been insignificant in terms of the total investment in that massive site. But the council was desperate to get RBS World HQ for Edinburgh (RBS was looking at sites in other countries too) so objections on things like how you'd get there by bike, which might have meant fiddly negotiations with RBS, were happily disregarded.