Perils of Void Travel
Cost and the length of time travel takes are not the only things to dissuade the populace of the Known Worlds from traveling between planets via the jumproads. There are other dangers to interstellar journeys, too - some very physical, and some that imperil the soul.
Many starships in the Known Worlds have been operating for hundreds if not thousands of years, and even those built more recently are constructed by shipwrights with an imperfect understanding of the technology they use. Consequently, air-reclamation systems regularly malfunction, radiation shielding is sometimes insufficient to the task, the heating system that keeps back the cold of space can fail, and any of ten thousand things can go wrong to the ship's engines.
Catastrophic failures are uncommon, but not entirely unknown; sometimes, they give those onboard enough time to be rescued, and sometimes they do not. Far more common, however, is key systems operating inefficiently or far below their required spec. The most common system for this to happen to is the air-reclamation system - indeed, it is standard practice on many starships to heavily supplement air scrubbers and related systems with the simple artifice of compressed, canned oxygen that is released into the system as people breathe. It is an inefficient means of providing air, to be sure, and sometimes has unexpected effects, but it can suffice for a journey from one planet to another before a ship can flush its air and bring fresh air in from the surface.
Another air-related problem is filtration. People are breathing the same air, basically, on a voyage that is at least several weeks and likely more. Any airborne disease brought on board will quickly spread throughout the passengers and crew. Starmen build up a tough immune system after some time aboard ship, but passengers who are not prepared at least catch the flu or a cold and often spends weeks violently ill from the centuries of disease that lives in the passageways of a ship.
Worse still is engine fumes and other unpleasantness that can get into a starship's air. This can start to coat the lungs of a starman, leading to a 'black lung' phenomenon not all that different than that caught by miners and others who work in places with heavy particulate matter. 'Spacer's Lung' doesn't usually affect someone after one voyage, unless the ship in question is a real scow, but it can be a problem for those who spend their lives aboard ship.
Cold and Radiation
Air problems are not the only problems interstellar travelers have to worry about. Cargo holds that have been converted into passenger space - common in 'stowage' accomodations - regularly don't have sufficient heating systems or radiation shielding. Indeed, heating systems can be a problem for entire ships, leading the crew to dress very heavily and the unprepared traveler to be very cold. In some cases, this can lead to hypothermia or frostbite for those not thinking about the cold.
A lack of radiation shielding can be deadly, but far more insidious is a low-level lack of the shielding - sometimes, a bad voyage can turn out to be much like chemotheraphy, with passengers beginning to lose their hair, get horribly sick, or just develop cancer. The worst part is that there is no real way to know if the shielding is good or not without an extensive check that's far beyond the understanding of most passengers.
The Dark Between the Stars
Finally, there are more nebulous concerns: space travel can be hazardous to the soul. In the deep void near the gates, whole ships have been swallowed up by Void Kraken, and ghosts have flooded the corridors to slaughter the living. The Church warns against travel in starships like it does against the use of all technology, and the wealth of stories about things that go bump in the dark between the stars makes many believe they may be right.