The cost of having to build new roofs over three stands negated the value of additional seating. The original plan was to have more seating but the need to support roofs meant large pillars were required rendering a large proportion of the additional seating having restricted views.
Whilst the screens have been crap for years it must be remembered that when the were initially installed they were state of the art and actually made good money for the club by renting them out to Wimbledon, cricket test matches, T in the Park et al.