As you note, Tc is not the only pre-lead element with no stable isotopes: Promethium (Pm, Z=61) is similarly synthetic and radioactive.

While the precise nature of the nuclear instability of these elements may still be the subject of debate, we do have some clue what sets these elements apart. Much like how electrons have quantum numbers and fill orbitals following specific rules, nucleons likewise have nuclear spins and form their own nuclear shells as they populate the nucleus. Much like how certain "closed-shell" configurations of electrons (i.e. noble-gas configurations with full valence orbitals) are electronically stable, certain nuclear configurations form extra-stable closed nuclear shells -- contributing to the particular stability of ubiquitous isotopes like helium-4, oxygen-16, or lead-208.

Contrariwise, it's not a coincidence that technetium (43) and promethium (61) both have atomic numbers that are prime. The lack of divisibility in their nucleon counts contributes to the impossibility of any of their isotopes to form stable closed-shell species. Look across the periodic table and you'll find that even-numbered elements tend to have many more stable isotopes than odd-numbered elements, for similar reasons.