The peer review process takes time.
And it serves to screen out papers of lesser quality (for example, if there are errors during the experiment or data-gathering stage).
Someone who is also a specialist in the field of your paper must review what you propose. If your paper contains calculations concerning a whole bunch of data, they must verify the calculations. They must also check if the data itself makes sense (they must repeat the experiment or, at least, some critical parts of it).
Just finding the proper specialists can take time. If you are well known, then they will scramble faster to find the right people, but that is because the journal knows it will gain glory from publishing your article. Otherwise, the process takes whatever time the process takes - months, years.
The paper must make a contribution to the field for which the journal publishes (translation = you must have discovered something new, or a new way to look at things, for example).
The paper must also be about something important for the readership AND be of quality (some journals receive hundreds of proposed papers every month, but only have room for twenty).