I wonder whether that's due to some economic reason (e.g., the Thai economy particularly sufferers from the lack of tourists), the unfortunate timing of Songkran (Thai New Year's national holiday, which the government may be reluctant to entirely cancel) or some other reason.
I cannot say definitively, but looking at the timeline I think it's mostly due to Songkran and poor timing. Regarding the timeline, I think two important events are linked to Songkran:
To see how these two events relate to the loose restrictions in the months leading up to Songkran, we will consider the promises from government officials about these events going ahead.
In early January 2021 there were minor outbreaks in Thailand. At the time, Deputy Public Health Minister Sathit reassured the nation that they had a chance to stop spread so the festivities 3 months later could go ahead:
However, if both state officials and the general public joined hands in containing the spread of the coronavirus, there is a chance the spread can be contained and people will get to celebrate the Songkran water festival in April, he said.
A few months later in March, with reported case numbers in the kingdom looking alright, the "secretary-general of the National Security Council, in his capacity as chairman of a Centre for Covid-19 Situation Administration (CCSA) subcommittee" said:
"At this point festive celebrations will be allowed this year," Gen Natthapol said.
From that point, Covid cases have been on the rise. This is not necessarily visible in the reported case numbers which only show a sharp rise starting in April. Nevertheless, knowing how the virus spreads one can only conclude that there must have been undetected local transmission as early as mid March 2021. This only became clear on April 7th when the Thong Lor cluster was found:
The first local cases of Covid-19 with the UK variant of the virus have been reported in the Thong Lor cluster, a noted virologist said on Wednesday, and warned it could spread faster than the kind previously found in Thailand.
By that time, the travel exodus for Songkran (started on April 12th) was getting underway. More importantly though, physical examinations for conscription were already happening that week. As such, any new measures would have to account for the fact that many people were away from home and would be (planning on) travelling back within weeks.
Also note that officials did not put a last minute stop to the celebrations. Instead, the message was to visit one's elderly relatives rather than to go partying as well as to keep in mind social distancing rules:
The Bangkok Post reports that Opas Karnkawinpong from the Department of Disease Control has warned of the need to adhere to disease prevention measures over the holiday. He says that while there is no ban on interprovincial travel, people taking part in traditional Songkran activities, such as merit-making and pouring water over Buddhist statues and on to the palms of the elderly, must do so in well-ventilated, outdoor areas, while wearing face masks and observing social distancing.
As for the economic arguments, I don't think that's a reason for lax measures. As we all (including Thai government officials) know, the virus spreads exponentially. So any benefit of lax measures keeping the economy going would cause case numbers to rise more rapidly. As such, that short term economic gain is no gain at all, especially when it comes to foreign tourists coming in.
If anything, proposals to reopen Thailand to tourists were all about reopening safely. For example there have been proposal to start travel bubbles with safe countries.
And a different plan is currently on the roadmap to (as I understand it) make the island of Phuket a large quarantine facility with all inhabitants vaccinated. The idea is that arrivals spend a few weeks on the island after which they are allowed to enter other provinces. I should say that a more pessimistic (and probably realistic) analysis of the Phuket plan may be found in yesterday's New York Times.
In conclusion, I think poor timing best describes how the virus regained ground in Thailand. By the time officials realized what was going on with the discovery of the Thong Lor situation, local transmission was already well on its way and it was boosted by those travelling to visit friends and (elderly) relatives.