Why there are 24 letters in Greek rather than 26?

You could ask why there are 26 letters in our alphabet. There's no set number for the amount of "sounds" that make up a language. The Egyptian alphabet has 45 letters. The Rotokas alphabet has only 12 letters.

Greek (24) and Latin (26) are close because we have such similar roots. You can even map Greek letters to Latin. Alpha, Beta, Gamma, Delta, Epsilon = ABCDE. Iota, Kappa, Lambda, Mu, Nu = IKLMN. Omicron, Pi, Rho, Sigma, Tau, Upsilon = OPRSTU.

Where a lot of languages differ is the F sound, J sound, the Z sound, and the "ch" sound. Some Latin things are almost unique, like Q. Similarly, other alphabets have consonant sounds that are unfamiliar to us.