I'll try to offer in unpolished form a few reasons why I think DCPS isn't particularly enamored of IB, some of which they might state publicly and some of which they might not. Some of these may be overstatements and some may just be off base, but they're what I think is going on.
1. It's apparently done very differently from normal courses, so you have to hire and train up a corps of teachers who are willing to put in the extra work and extra professional development time to do it, and they have to stay committed to one school for some serious time to make it really go. DCPS already has challenges getting and keeping staff.
2. While IB sounds great from a superficial first glance, it doesn't seem to add a tremendous amount of value within the context of DCPS. To be blunt, they put it into Eastern, etc., and for what? It's harder for everyone and it doesn't seem like any educational magic has happened.
3. There are the costs and ramp up time, as others have mentioned.
4. DCPS has other ideas for programmatic improvement to benefit student outcomes that don't involve making this radical a change in teaching and learning.
5. DCPS believes they can implement global education and dual language programming without having to sign up for the complexity of IB.
6. The people who are asking for it are a tiny minority who typically vote with their feet to not attend the schools where they suggest expensive, disruptive new programs of dubious benefit to the majority of students.
7. DCPS folks believe that parents and local homeowners asking for it are just looking for a proxy for quality, know little to nothing about the program, and DCPS teaching and learning is fundamentally sound (with the unstated implications that motivated students respond well to it in the right settings (i.e., Tenleytown) and unprepared students will fail or avoid IB, particularly where the gap between current student ability and challenge presented is yawning).
8. Those asking for it are effectively, if not openly, asking for school-level segregation/tracking instead of inter-school segregation, and possibly even magnet-type separation from the community. At minimum, they are seeking a way for some children to stay in an advanced track or tracks separate from the majority who are not at grade level and minimize interaction, at least on an educational basis.
9. Maybe - maybe - they think that niche educational programming at odds with the norms for DCPS is one of the reasons for charters and they should do that kind of thing, not DCPS, which has to shoulder the burden of educating those who don't want niche programming.
I'm trying to lay out other folks' thinking, so it's pretty likely some of this is not right. But I think these are all things people should recognize if they want to make IB a priority for new school programs. I think there are a lot of positives that can be said about IB - this article was inspiring to me -
http://www.seattletimes.com/education-lab/stunning-surge-in-graduation-rate-as-rainier-beach-gamble-pays-off/ but I don't realistically believe DCPS is going to go down the road to IB at MacFarland (or Roosevelt) of its own accord.