All of the following is purely my opinion, based on observation from having watched Thai dramas for years too.
Because lakorn is a template - so it must have p’ek, n’ek, and rai roles. So whatever script or characters, they must be slotted in these roles. To me, this is the biggest contributor to the lack of diversity in Thai dramas. Because all scripts must fit in this lakorn format, it is very limiting. No matter how some producers try to change, in the end they stick to this basic format. Which is why we have also gotten so many which seem to start out promising, but halfway descend to the typical annoying lakorn plot nonsense.
Thai actors/audience themselves differentiate dramas this way. e.g. Sunny has specifically stated he would act in drama series, not lakorn. Series - dramas with no such clear cut roles like those in lakorn. Basically most Thai dramas I see on Netflix from GMM or True.
The industry also has a hierarchical view of actors based on their status as p’ek/n’ek and supporting roles. For example, supporting or rai roles is viewed as a downgrade, while p’ek/n’ek status is viewed as the holy grail which all actors seek to attain. The truth is for most good actors, they would just want good roles. Because there can be supporting characters that are more memorable compared to lead characters, but lakorn format would not enable that.
Regarding actor choice for lead roles, channel/production company politics also play a part. Connections help a lot. Just see how Ae Supachai managed to launch so many of the current stars in the industry. And channels like Ch3 also enable this when they simply pick artists with huge fan bases for lead roles, rather than using a casting/audition process to find the most suitable actors for roles.
If we use the American context as examples, lakorn = soap operas, unlike series which are like regular dramas. And today’s viewers want to watch dramas, but the Thai channels are stuck in lakorn but don’t seem to want or know how to evolve.
Entertainment industry in every country has their issues. This is just my opinion on what’s happening in the Thai one.