Perhaps because the time period in which Tom Riddle performed his first acts of crime were concurrent with the time period in which Grindelwald was still gaining power, and from what we have learned in Fantastic Beasts: The Crimes of Grindelwald, there are people like Credence Barebone (Grindelwald's weapon) and Grimmson (a bounty hunter who takes orders from both the British Ministry of Magic and Grindelwald. So, quite literally, an INNER) who posed a threat, a secret threat in the case of Grimmson, against Britain. Who knows, maybe Grindelwald told Grimmson to Imperius other wizards in Britain during Grindelwald's reign? We also knew that even the members of the British Ministry of Magic were afraid of Grindelwald. Grindelwald easily dealt with more than a couple dozen Aurors with ease, and none of his followers were killed by his enemies, save for the girl who got shot with a Killing Curse. Also, none of his followers fought. As such, magical Britain might have been in a similar state to what it was in the First and Second Wizarding War as even the Aurors had a hard time. Also, that similar ministries of magic had fallen already against Grindelwald.
Here's an excerpt from Half-Blood Prince:
— — ISSUED ON BEHALF OF — — The Ministry of Magic PROTECTING YOUR HOME
AND FAMILY AGAINST DARK FORCES The Wizarding community is currently
under threat from an organization calling itself the Death Eaters.
Observing the following simple security guidelines will help protect
you, your family, and your home from attack.
- You are advised not to leave the house alone.
- Particular care should be taken during the hours of darkness. Wherever possible, arrange to complete journeys before night has
- Review the security arrangements around your house, making sure that all family members are aware of emergency measures such as Shield
and Disillusionment Charms, and, in the case of underage family
- Agree on security questions with close friends and family so as to detect Death Eaters masquerading as others by use of the Polyjuice
Potion (see page 2).
- Should you feel that a family member, colleague, friend, or neighbor is acting in a strange manner, contact the Magical Law
Enforcement Squad at once. They may have been put under the Imperius
Curse (see page 4).
- Should the Dark Mark appear over any dwelling place or other building, DO NOT ENTER, but contact the Auror office immediately.
- Unconfirmed sightings suggest that the Death Eaters may now be using Inferi (see page 10). Any sighting of an Inferius, or encounter
with same, should be reported to the Ministry IMMEDIATELY.
For these reasons, the Trace may have simply been suppressed or removed during the Global Wizarding War against Grindelwald. Therefore, Tom Riddle's magic may not have been sensed by the Ministry as they had more trying events in their hands, and by the looks of it, in as early as 1927, the multinational Aurors were already losing.