In a word, Politics. The current CEO of F1 Stefano Domenicali, had a planned meeting with the PM the Tuesday after the Australian F1 weekend (18-Mar-25), to which he commented the plans were "impressive", and that the Thai Government would commission a feasibility study into hosting a Grand Prix street circuit from 2028.
No point doing anything before 2027, as that's when Boss's statute of limitations expire. So Red Bull will be out in force.
Also, Stefano as CEO of F1 is technically responsible for F2, F3 and F1 Academy. So reading between the lines, what might be constructed isn't necessary a F1 track to F1 FIA standards.
Taking the Singapore F1 circuit as an example, the initial 2008 race cost 150 million Singapore Dollar, or 3.8 Billion THB at the time. Of which the Singapore Government stumped up 90 million, with ongoing running costs, approx 20million Singapore Dollar pa. Initial contract was for 5 years, so ROI of 38Million pa to initially break even. Luckily the Singapore contract has been extended, and is currently contracted until 2028.
Has Thailand got this kind of money, especially with other infrastructure projects, ie the canal, high speed railway, Bangkok overhead road etc.
Could this be a coincidence, Singapore 2028 vs Thailand 2028, or is F1 playing politics with a possible alternative race in SEA to fleece Singapore for more dollars and keep the night race on the calendar for another contract extension?