Why is the Boeing 747-300 not in service anymore?
The 747-300 was somewhat of a cross between -200 and SP. The -300 was almost same as the -200 except for the increased length of the hump. The -300 was never a success and was soon overshadowed by the -400. 

The 747SP was the result of a requirement for longer range aircraft. Boeing engineers basically cut around 47' from the 747 fuselage in SP. However, by the time it flew for the first time, commercial air travel had changed (in no small part due to the high oil prices) and the model never sold as much as Boeing had hoped. However, when it was observed that there were no adverse effects when the rear of the cockpit lined up with the front of the wing, Boeing introduced the 747-300 with increased length of the hump with little else in terms of major change from -200.

However, just two years after -300 was introduced, Boeing introduced the -400, which was a significant improvement- it eliminated the flight engineer, increased range among others, while retaining the -300's hump (which meant more economy seats). 747-400 is the most successful model of the series, with more than 690 orders. Most airlines jumped straight to the -400, effectively killing the -300.

Actually, the 747-200 production (with over 390 made) ended after the -300 version (of which only 81 were made). So they are more of contemporaries. 

Also, it'll be a stretch to call that the older 747 versions are still in passenger service. 747-100 is not flown with passengers anymore, current operations of 747SP hardly qualify as passenger service and the most successful of these, the 747-200s are also being slowly phased out, if you discount the VC-25s flying around with US President (which are slated to be replaced by 747-8s).