A variety of factors, mostly the amount of policy and paperwork involved making it unnecessarily difficult to book range time and ammo.
I think it has more to do with ammunition entitlements (cost) that units manage to justify and receive rather than excessive paperwork. Rather, the problem is the ammunition entitlements received by ordinary units - paperwork, especially nowadays is just a matter of cut and paste date changing, and a risk plan is a risk plan be it tomorrow, next week, or next month.
In the past I worked with units that had an on paper entitlement to around 10k rounds per man, per month, if they wanted it. That along with their standing range bookings saw all available personnel attending three perma-booked range sessions per week. Most of those personnel got through more ammo in a morning than most squaddies would get through in a year.
The bottomline is cost, always has been, always will be. No one at MoD is going to sign off on every squaddie having 100 rounds per week, every week, not even 50 rounds per week I'd wager. As a civvy shooter I know my fullbore shooting skill degrades after 5 days, no matter how much airpistol, or dri-firing I do. I also know that a half meaningful practice needs at least a hundred rounds thrown down range.