Omaha is a Texas Hold’em variation played almost identically with the only exception – the player is dealt 4 hole cards dealt face down instead of two as in Texas Hold’em.

Remember, it’s highly important to keep in mind this fact, as in case you don’t, you are likely to have some troubles with cards volume definition. Take a look at this vivid example:
Your hole cards are: 5h – 10s – Js – As.
The community cards are: 3d – 4d – 6c – 7c – Kh.

You may get surprised but here, neither you will make the Flush with Ace high and nor the Straight with 3-4-5-6-7. In this specific case, the only best combination to happen is a hand with Ace high.

And here the best answer for your why will be the following: In Omaha Hold’em your combination is created from only two of your hole cards and three out of 5 on the table.