cost,cost & cost
Above the reason why the C152 is such a good trainer, yes the PA38 is a better trainer and the Robin is far nicer to fly but the objective is to teach people to fly at the most economical cost the goal being the issue of PPL.
Reasons for not using the PA38 are that parts are starting to get a bit hard to get hold of and 13,000 hour spar life so a high life aircraft will have next to no resale value so don't expect to be flying anything except an old dog by now.
The Robin is by far the nicest aircraft to fly and a very good trainer but Robin are by far worst aircraft company at suplying parts that I have ever delt with and dispite being a long time DR400-180 owner I would never try to make money with the aircraft and so regretably it is a non starter as a trainer.
The C152 dispite being out of production is an aircraft that has unlimitted spar life and a good parts suply chain, being a "beefed up" C150 it has had all the bugs of the C150 sorted out and so maintenance costs are the lowest of any of the popular trainers so it is an aircraft that an owner can spend money on with confidence of a return on investment.
The C172 is a fine aircraft ( but not as nice to fly as the Robin) with all the advantages of the Cessna parts supply chain but as noted in other posts you are flying two empty seats around and I'm sure Ryanair would not do that as a matter of policy!!