There doesn't seem to be a clear official statement regarding the delays in counting the vote.
But, the fact that the election itself was delayed by about 39 months (from an initial promise of the end of 2015 to March 24, 2019 for domestic voting) is suggestive of the situation. So is the following description of the delays in the vote counting via Wikipedia:
The Election Commission drew much criticism for problems with the
election. These include issues with overseas voting, where poorly
designed envelope labels led to mailed ballots being rejected or
mis-delivered by postal services, and information pamphlets contained
unclear or incorrect information. On election day, it became
apparent that 1,500 ballots from New Zealand were misplaced during air
cargo transfer and could not be delivered to counting centres in time
to be considered valid. The polling process saw many reports of
irregularities, and the counting process and initial results were
very confused, as the live figures released by the EC contained large
amounts of errors. Unofficial results, which typically would be known
by the same night and announced next morning, were repeatedly
Wikipedia cites to the following sources for these conclusions:
46 Achakulwisut, Atiya (19 March 2019). "Poll agency does fine job of not inspiring trust". Bangkok Post. Retrieved 26 March 2019.
47 Yuda, Masayuki (13 March 2019). "Thai election regulator comes under fire for irregularities". Nikkei Asian Review. Retrieved 26 March 2019.
48 "Late ballots from NZ delayed for days at air cargo warehouses". Bangkok Post. 26 March 2019. Retrieved 26 March 2019.
49 Charuvastra, Teeranai (26 March 2019). "EU, UK Urge Thailand to Resolve 'Election Irregularities'". Khaosod English. Retrieved 26 March 2019.
50 "Thailand election results delayed as allegations of cheating grow". ABC News. 25 March 2019. Retrieved 26 March 2019.
Another important factor is that this is the first election that has been conducted since the military coup in 2014 imposed a military regime, and this election is being conducted pursuant to a new constitutional with a mixed member proportional system that is much more complex than the previous first past the post single member district system elections in Thailand.
As a point of reference, the U.S. state of Maine adopted a ranked choice voting system for the first time in the 2018 general election, when all of its previous experience was with a first past the post single member district system, and this greatly delayed the amount of time it took to release the results. This delay occurred even though Maine has a two centuries long history of administering elections without interruptions for regime changes, or other factors adversely impacting the efforts to the Election Commission in Thailand.
In short, while there are plenty of potential sources of malice to point fingers at, incompetence on the part of the administrators of the election administration system in its first reboot of electoral democracy in Thailand using a new electoral system, clearly played a part as well.