The standards for accreditation are much higher for LCME schools than it is for AOA. It will never happen because it costs tens (if not hundreds) of millions of dollars to hire 2-3x the number of faculty (I've heard as much as 6x in one article of MD faculties vs the average DO school). DO schools get less state funding. You also have to keep in mind the caliber of DO students is much lower (I'm pursuing ONLY DO myself so this is just the reality, not bashing the profession. I know that many external factors can account for lower GPA/MCAT).
Anyways, there's a reason that a 3.2/500 can get you into a school like BCOM/KYCOM (22 avg in 2011 according to wiki ~492 today) etc. The chances of passing COMLEX with stats like that are much higher than passing Step (using PR or HBCU MD for example, first-time pass rates are 80-90% and many graduate in 5-6 years). LOTS of DO students do not take Step and many would fail if they did (your DO school might be the exception so if you're at a PCOM/KCU/CCOM you're an outlier where a higher percentage take/pass step). If all DO students were forced to take Step you'd see a 30-40% failure rate across the board, 20% at the more competitive schools, and as high as 70% at your newer/"lower tier" DO schools. If there are individuals happy with taking just COMLEX and matching rural FM then just let it be.
If DO schools suddenly became MD (and thus more coveted) it would be very difficult to attract competitive applicants who can pass Step (let's say 3.5/504 as a confident number for passing - I know MCAT doesn't correlate with Step but just look at most Step failures/non-takers and you'll see the correlation, and I can discuss the data in more detail but that's for another discussion) and in that case, you might be tempted to recruit more from OOS or bigger nearby cities, and exclude the rural kid who didn't have many opportunities (and this 3.2/500 applicant is MORE likely to serve his rural community than moving to LA/NYC). Also, competitive applicants are less inclined to pay 50-70k/year tuition to live in less desirable cities (most DO schools).
Not to mention new/mega DO schools opening up left and right, or their desire to expand their classes to ~500 students. This wouldn't fly with LCME, and there are incentives for the respective organizations to allow this to happen. Personally, I'd prefer training more DO students interested in IM/FM than giving those spots to IMGs.
Just my thoughts.