When asked why a gamer lapsed, the answers (multiple choices allowed) were:
Got too busy with other things: 79% Too few people to play with: 63% Not enough time to play: 55% Found a game I liked better: 38% Unhappy with the game and the rules: 38% Cost too much money: 32% Burnt out from frequent play: 29%
While 3 editions have been released since this survey potentially solving a couple of those issues, there still are some ways to solve these concerns without having to wait for new editions.
Getting too busy with other things/Not enough time to play
Getting busy with things happens and once and a while that will interrupt your gaming. Last night, one of the campaigns I take part in was cancelled because 4 of 7 people were studying for finals and working on final projects. A few solutions to solve this would be plan sessions a little farther apart, but play for longer. One of the campaigns I run meets only about once a month, but we play for 5-7 hours in a session. Since most of the PCs are students, I have to be flexible for their availability and if only a few people miss then we still meet up (though this party has about 10 people). If you yourself are busy and a DM, it isn't always a bad thing to cancel a session here and there, but you need to make sure that you don't do that too often if possible.
Too few people to play with
Depending on your comfort level, there are a few groups that meet online on sites like Roll20.net that actively have "looking for party" or "looking for DM/GM" messages out there. If you really want to play and are comfortable playing with strangers, it could be really fun and simple to join one of their online adventures. If you're not as comfortable with that, you can always run small party campaigns. In college, I ran a one person campaign with my roommate where his character did have to deal with some tough situations. Eventually that campaign grew to 2 people, but really what it comes down to is are you or your DM comfortable with the amount of players in the campaign.
Found a game I like better
A seemingly impossible thing to get around because if you don't like D&D, then it may be hard to bring you back. However, if you're looking to trying D&D again, think about the aspects of the other game that you like best and try to see if they can be incorporated into a character or perhaps into a campaign if you are DMing.
Unhappy with the game and the rules:
The game itself is based on the storytelling of the DM usually, so perhaps it isn't the game you dislike, but rather the style that your DM uses to tell the story. If you don't care for the roleplaying aspect of the game, then you could always make your character more of an introvert and only speaks when they have something important to say. As for the rules, if you don't care for a rule, ask your DM if you can change the rule and make it a house rule. Nearly every party that I've DM'd for or have played in has had some list of house rules.
Costs too much money:
The only amount of money I have put into D&D has been because of my love of the game. I started playing with a DM who was willing to teach me without really having to look at any of the books, but he had them available if anyone wanted/needed to look something up. As a player the only money I put into it was by my choice to by some dice so I didn't have to borrow any and that was pretty much my only cost to play. I can understand that if you want to buy modules and all the other books that the price can get pretty steep, but when I DM, I don't use any premade stories because I prefer to develop my own as I go along. So really, I see D&D as something that could cost a lot if you want to buy all the books, but it is just as easy to not worry about the books as much and learn as you play and then if you enjoy yourself consider investing in the books for yourself, or even simply borrow the books from someone.
Burnt out from frequent play:
I can understand this one, generally after a week where I DM 2 campaigns and play in one, the following week I feel burnt out of ideas and the sessions tend to be a little poor or get cancelled because I don't want to DM when I'm not fully into it. Generally, I try to give myself a break every so often whether it's skipping a week or having a simplified week so I can better develop my plans out. For any campaign, I think it is probably best to only play once a week for about 2-4 hours that way you can get some stuff done, but you leave your players wanting more. On the other hand, if you are burnt out as a player, perhaps talk with your DM and see if you can have your character go away on a personal mission for a few months and bring in a second character that fills in while your original is away. This will allow a new dynamic to enter the party and could allow for the party to change a bit. This doesn't mean that character is there forever, but it could provide you a little relief if your play gets a little stagnant.
When a game gets hard to play, it is time to see what makes the gameplay unenjoyable and see if you can change it. Chances are you will be able to fix it and you can resume enjoying your time again with a few tweaks here and there. Don't be afraid to test something and look for feedback, sometimes changes don't make things better and you might need to revisit the plan.