The TR specifies a 4v/cyl 90° V-8 with circular pistons/cylinders in an effort to restrain developmental costs. History has proved that some teams will spend cubic Euros to gain an advantage by exploiting any property not explicitly defined to the most minute detail. For instance, the TR only specifies 99% of the ingredients in the petrol the teams must run, so Ferrari employ 40 people to determine what to do with the remaining 1% to produce optimal engine performance. I'm not sure Hispania Racing have a total staff of 40 but Ferrari have that many just tweaking fuel.
A V-10 is a mechanical abomination because a straight-5 is inherently unbalanced (both primarily & secondarily), and a V-10 is two abominable 5s joined at the crank. F1 embraced it when displacement was raised to 3.5 litres because 300-350 CCs per cylinder is optimal for thermodynamic efficiency (350cc X 10 cylinders = 3.5 litres). Engines then only had to last the one race so reliability wasn't as strong a concern as it is now. The 90° V-8 has perfect primary and secondary balances so it will tend to have better reliability.
When limited to 4 valves per cylinder (as per the current TR), the V-10 will have a breathing advantage over a V-8 (40 valves vs. 32), but the V-12 will breathe even better (48 valves). Plus the straight-6 has perfect balances and two sixes mated at the crank -- regardless of the angle -- is smoother still.
The current engine configuration was set in 2006. Displacement was reduced to 2400CCs to try to slow the cars down a bit. A V-8 is cheaper to build than a V-10, plus no two designers can agree on the proper bank angle for a V-10. But no one disputes the benefits of the 90° V-8, so the FIA chose it so there would be no flexibility in the design. Otherwise, Ferrari might have spent a gazillion Euros deciding whether 110° is best or 120 (or anywhere in between).
I am fair certain neither BMW nor Porsche would have resorted to building V-10s for the street if F1 hadn't been running them. It's an exceedingly compromised design and road cars with them need more counterbalancers than Paris Hilton has had social diseases just to dampen the vibration.
You can keep the V-10s but I'll take a V-12 any day.
It's also about costs and about applicability for passenger cars.
Get used to the idea of V6 F1 cars in a not too distant future... 
My money's on the turbo straight 4 with regulated boost and unlimited revs.