Let's face it - at times, our railways simply don't cope!
It's more than "at times", though, isn't it? In fact, we may as well say that our railways cannot cope with the surge in demand that the cynical may suggest is the opposite result to what the government and other bodies and organisations expect when they continually raise fees.
With regards to stations other than major terminals and also stations in the South-East, I am appalled by the lack of platform lengthening where demand for longer trains is often shown - and where geographical constrictions are not notably in evidence.
I think this is one of the key barriers to the use of greater numbers of carriages (which, let's face it, is often going to be the only sensible solution, especially when pathings and so on are restricted (e.g. due to engineering works) and a comparatively small number of trains can be run).
The other major barrier is the time it takes to build and supply carriages to TOCs to meet forecasted or actual demand, or else to move stock around the country before it can be used. It can take an inordinate and ridiculous amount of time to acquire new multiple units, carriages and locos - partly due to EU procurement and bidding rules, partly due to a lack of efficient larger-scale British manufacturing, and partly because of all the longwinded administrative processes between and in different organisations that must be slowing things down by orders of magnitude. We talk about projects for building stock stretching many years in the future when it will probably be more efficient to predict the short term and build stock quickly for the short term if necessary (it can always be sold on or kept in good-quality storage if unused for a few years, but this seems hardly likely).
I understand there are other factors, such as maintenance schedules, asset management routines (most of which can probably be made more efficient), training for new stock types, building regulations, planning permission, etc. etc.
Now, of course, the content of my post has been stated many, many times before, but it bears repeating, and I feel this is a good simplification which government departments (etc.) may understand a little better!
Perhaps if we want an even simpler answer, we can go right back to LHCS for everything, which would probably enable better tidal flow capabilities and more flexible maintenance as necessary.