Lots of stuff
One reason solar panels aren't made so that they can receive sunlight on both sides is that that would, essentially, double the cost (Solar cells are still relatively expensive, at least the radiation hardened, space qualified variety.). And, any time you double the number of parts, you increase the chance of a failure. Plus, even doubling the number of solar cells (via front/back installation) doesn't cover the case where the thing happens to be flying sideways.
There's also a weight issue. It's VERY expensive to send mass into orbit, and even more so to send it to Mars (even using a Hohmann Transfer Orbit). Thus, spacecraft designers are paranoid about adding even the slightest amount of non-essential mass to a spacecraft, since that often means 100 times (or more!) fuel is required to boost that additional mass into orbit. And, it doesn't take too much before the size of even the largest launch vehicle is exceeded.
Ok, so why don't they make the satellite a sphere and put solar cells all around it? That has been done with some of the smaller probes/satellites, but it still relatively cost inefficient, and has some geometry problems for larger probes/satellites (e.g., Spherical probes won't fit on the top of the rocket, under the fairing.).
The best option is to usually install some type of sun sensor, with a motorized mount to rotate the panels (or the craft) so that the solar panels are facing the sun. That was obviously what was intended here, but something went horribly wrong. :-(
Of course, it's not trivial to put an electric motor on a spacecraft. Motors tend to need lubrication (else they seize), and you can't just slap a bit of axle grease on them, since hydrocarbons tend to do VERY nasty things when subjected to the extreme environmental conditions in space (e.g., high/low temperature extremes, zero air pressure, zero gravity, ultraviolet light/x-ray/cosmic-ray exposure, etc.). There are things, such as some Graphites and some Teflon,which can be used, but even those have to be used with a bit of care.
Of course, this wouldn't be the first time something has went wrong with a space probe. Don't forget the American Hubble Space Telescope that was near-sighted (Ok, so it suffered from spherical aberrations, due to the measurement system used when it was ground having been assembled backwards.). And, there were many, many others. :-(
As for the chance of a rescue mission, that never really existed, even given the fact that the American Space Shuttle was used to retrieve/repair/return a satellite or two. One problem is that the Space Shuttles are no longer flying (They've all been retired to museums.). Another is that the Phobos-Grunt probe is flying VERY low, so low as to make orbital perturbations due to atmospheric drag a very real problem (and, trying to synchronize orbits between two spacecraft is hard enough, even when you don't have to worry about the orbits changing due to atmospheric drag). Plus, there's the very real concern that there are many thousands of kilograms of VERY energetic fuel on board the probe, and it's uncertain what the condition of the satellite is in (e.g., Is it leaking fuel? Are the valves damaged? Is the piping/tanks intact?). Would you want to fly up to a 8000 Kg bomb at 17,000 miles per hour, and hope that everything was in good shape?
Despite the professionals desiring it to be believed that space travel is easy, it isn't. It's VERY, VERY, VERY difficult.
P.S. I'll get my coat; it's the one with the OSCAR satellite in the pocket.