Quick look at the mustang mechanically.
They make great drift cars if you set them up right. But it takes alot of time, money and engineering to do so. Their weight is a big problem (not in the 93 and earlier models, their pretty light!) and it is really hard to get it down if the car is to be still daily street driven because alot of the things that make the chassis safe and strong have to go. Everything that can be, must be fiberglass or lighter. The distribution of the weight is another huge issue. The typical rotational axis of the mustangs is generally right behind the engine. This makes it VERY nose heavy and gives it the tendency to want to loop around on you all the time. So you need to address that. I am currently sitting just over 4000lbs, WAY heavier than a 240. The easiest way is with moving what you can to the trunk (like I have an entire cooling system back there holding 5gal of water, helped ALOT!) and lightening up the front with a new K-member and LCA's (Those are only $1000 tho, no biggy right?) Now that were on topic of suspension... Coil-overs are a must, but are very expensive compared to the 240s. I paid $2000 for a 3rd best brand on my wish list. Thats just the coil-overs! Now your talking alot more $100-$200 hits here and there for the rest of it, front and rear LCA's($350), K-member($650), Bushings, Swaybars, Tie-rods. Brakes?? You cant touch a decent new big brake (14") kit for less than $2000 (FRONT ONLY) You dont really need them in 94+ unless you get real serious because upgraded stock parts will do, but just letting you know. Now if you have 93 or earlier you need to get 94+ spindels and axils to get 5 lug. Can be cheap if you junk yard it but you wont find anything to pull off because its all gone by now. and its not just putting the brakes and whatnot on. The 93 earlier master cylinders wont work with the newer brakes so that has to be updated to. Plus the suspension for the older cars if a tad more than the newer stuff. Now the biggest draw back to the 2004 earlier mustangs is the angle... there is none! With a 7" wide rim backspaced out away from the frame as much as I could get without special ordering I have modded my rack and my spindles and with spacers I am hitting my frame. I have close to if not less than what a 240 has stock. I have to hit every entry dead on the money because I am riding almost full lock the whole drift. Theres no room to fuck up because you will spin. Everything about the car is fighting you the whole time. The power helps but with the cars mid drift stability in question it can hurt more than it helps when your learning. You cant just throw it in a turn and if you go in to hard just turn the wheel some more and correct it because there no more to turn. Its an uneasy feeling to have doing 90mph+ entries at PBIR. Now 05+ mustangs are a whole different story. They were made for it. Power, somewhat affordable suspension, decent chassis weight, and lots of angle!!! Their stock angle is way more than my car modded and when you start playing with it its comparable to a 240 modded. They are a very good drift platform, their just newer and more expensive to buy. So said and done it takes time and patience to work on and learn how to drift a mustang and its a piece of cake to drift the 240s. So someone hops in a mustang finds out it harder then hops in a 240 and in comparison the dame thing almost drives its self imo. Ive drifted several, the 240's easy to drive so thats why its so popular. This is just my look on the situation having delt with my mustang for 4 years now. Oh one more thing Ill take the IRS over the Solid axle any day of the week. Properly setup it is so much more stable and predictable in the higher speeds. Just one more thing to have to upgrade if you dont go cobra from the start. Now I'm by no means saying its not possible or trying to stray people away from it because I've been doing it for years I'm just answering the question of why yo dont see alot of them from a mechanical standpoint.