Why have Monarch got limited training resources? They're a TRTO after all.
For the same reason that Oxford university (another training organisation) has limited vacancies each year. There are many other demands on an airline than simply type rating new applicants. The number of training captains available, simulator slot availability, aircraft availability, safety pilot provision. This is just a part of the constituents used to make up available training tracks. Those tracks then have to be allocated to internal fleet changes, promotions, combinations of the former. Add to that the intake of new cadets which places a heavy demand on those tracks, together with the vast number of routine LPC/OPC renewals, Category C training and checking etc.
The economics of how you match supply to demand doesn't allow for spare capacity that would simply be uneconomic outside of the peak (winter) recruitment and training season.
For an airline taking on experienced type rated people, the demand on the training tracks should be minimal. With cadets, the demand on those tracks is very high, but is offset by other economic benefits. Obviously in the middle (non type rated but otherwise experienced pilots) it is a very squeezed market. As fmgc has said, that may eventually change, but it is a fact of life in the current marketplace.