Short answer Because of the fixed law of time in Jain cosmology: in every half‑cycle of time (each Avasarpini and each Utsarpini), exactly 24 souls endowed with Tirthankar‑nāma‑karma take birth and establish the ford (tirth) for liberation. So, per half‑cycle there are 24 Tirthankars—no more, no less.

Slightly longer, in simple points

• Time moves in endless cycles, each with two halves: Utsarpini (ascending) and Avasarpini (descending). In each half‑cycle, 24 Tirthankars appear.
• They arise when living beings especially need guidance—primarily in the 3rd and 4th eras (ārā) of a half‑cycle—restore dharma and establish the fourfold saṅgha (monks, nuns, laymen, laywomen).
• The number 24 is not a human proposal; it is the eternal pattern revealed in Jain scriptures and by omniscient Kevalins. It is tied to very rare karmic fruition called Tirthankar‑nāma‑karma; only 24 such souls manifest in each half‑cycle.
• Many other souls attain moksha, but “Tirthankar” is a special role among the 63 Śalākā‑puruṣas; 24 of these 63 are Tirthankars.
• This repeats forever: in every past and future half‑cycle there are 24 Tirthankars. In our current Avasarpini, the last (24th) was Bhagwan Mahavira. In the next Utsarpini, again there will be 24.

Digambar and Shvetambar Both traditions fully agree that there are exactly 24 Tirthankars in each half‑cycle. (They differ on some historical details like Mallinath’s gender, names of parents, etc., but not on the number.)

For more on the time‑cycle and eras: