why dont iron particles get stuck to the mid of bar magnets when it is the strongest part of the magnet

It is very true that Magnet's middle portion is the strongest , but only from inside (like in solenoid). Degree of attraction of particles depends on the magnetic fields present in a region . But the magnetic field present in the middle region of a magnet is the least and thus attraction of the iron particles will also be the least! So I hope it's clear to you now.!! Refer to a fig. of diagram with fields shown on it. It may help you more effectively.