I actually made a very lengthy comparison thread and some videos between the EX1000, 7550, XBA-4 and XBA-40 for anyone who is interested:
http://www.head-fi.org/t/608553/review-comparison-two-flagships-pass-in-the-night-sony-xba-4-review-vs-the-mdr-ex1000-updated-with-7550-and-xba-40
But to answer the OP's question, I have often wondered why the 7550 is less popular than the EX1000 myself, and I think it comes down to a few things:
1. The 7550 was never marketed the same way the EX1000 was - it was geared towards Sony 'Pro' users for use as a professional monitor.
2. Marketing was split between the 7550 moniker in US markets and the EX800ST moniker in Japan / Asia, which probably made it confusing and less likely for any word of mouth to spread between domestic markets. When the EX1000 and EX600 was released in Asia, the EX800ST was almost a mystery footnote release. The 7550 only appeared on the scene quietly in the US. I do not even know if the 7550 / EX800ST is available outside of Asia / US markets. It is not available for instance in Australia. (Nor was the EX1000 or EX600 I believe).
3. I think when you are looking at super expensive premium gear (and the EX1000 was stratospherically expensive at launch) strangely enough no one really talks about the 'also ran' models in the middle of the spectrum unless there is a really compelling reason to do so. Everyone likes the budget models because they promise to capture the magic of the high end at a bargain price, and everyone likes the flagship models because they are aspirational. The 'also ran' tends to suffer in my view because it is not priced low enough to attract the 'HEAD FI BARGAIN OF THE CENTURY' punters but not high enough to catch the fancy of the top fliers.
For instance:
AD700 / AD900 /
AD1000 / AD2000 - As far as I can tell there is very little information about the AD1000. The same goes for the X series.
XBA-1 /
XBA-2 / XBA-3 / XBA-4 - This is an interesting case because the XBA-4 is almost universally considered terrible, so the XBA-3 almost became the flagship in this case. The XBA-1 is considered a well balanced earphone and the XBA-3 is considered well balanced and no one seems to talk about the XBA-2.
Now as for the 7550 itself, I love my pair. I would really hesitate to say that either the 7550 or EX1000 is better, because they are magical in their own ways.
The EX1000's build quality really screams Sony super premium and the sound seems to match that ethos. The signature works wonderfully for all the kinds of music we associate with 'audiophiles' - slow, atmospheric, female vocals, etc. (This is also why I hate the term 'audiophile' because of the stuffy connotations).
The 7550 is very much more a workhorse IEM. It works a lot better for a lot more genres (I actually used it as a monitor myself) and is an everyman. However, here's the rub: the 7550 doesn't have the same kind of romance attatched to it in either the sound, build quality or price. It is just a very neutral IEM.
Compared to other neutral IEMs like the GR07 and RE-400, it comes off badly just because of it's strange fit and poor application for portable use. (Wind noise / poor isolation). Unlike the EX1000 though it doesn't have the same kind of magical unicorn audiophile assocation that renders its flaws as esoteric and charming rather than inconvenient design.
That said I like the 7550 for its soundstaging and its rich tone - they make me think of the EX1000 without making my ears bleed for it.
Also keep in mind that different people are sensitive to different frequencies. Some people might find the treble spike of the EX1000 hunky dory. I think the EX1000 sounds great until I try to play any modern recording on it, at which point it just gets very hit and miss.
I reject the idea that the EX1000 only sounds bad on bad recordings, because it's a somewhat facile argument - it implies that somehow the music is bad because the sound engineer done goofed.
I would compare it to sports suspensions on sports cars: they offer great handling and a good feel with the road surface, but are extremely jarring when travelling on any uneven or badly maintained road. You wouldn't say: don't travel on bad roads then, because people actually need to get places. The 7550 is a comfortable ride that gets you through a lot of music wonderfully. And of course, every now and then when the road is right, it's fun to take the sports car out for a spin.
If Sony had made an EX1000 with variable tuning so that you could get the same 3db low pass filter with the EX1000's build quality and low distortion driver characteristics I would really fall in love.
