Whenever an airline introduces a new type of aircraft, it incurs a huge expense in terms of training, software systems, tooling, parts etc. For them to discard that type and move to something entirely different is only viable if someone else - eg the manufacturer - is prepared to pick up the tab.
As it's based in a Category 2 country, BeeWee cannot get the ETOPS certification it requires to operate to Europe. This means that the only games in town for it are the MD11, A340 or B747.
In any event, operating an ETOPS standard aircraft is extremely expensive, in terms of the increased staffing required as well as considerably higher costs for parts stocks and maintenance.
There is about as much commonality of the CFM56s powering the A340 and those powering the B737-800 as, say between the RB211-22B and the -524B4; which is to say about as much between the -524 and the Trent!