Hi! Well done on reading so far in advance!! :)
The marketability referred to here means the ease of buying and selling at the price you want with minimum dealing costs.
Government bonds are issued in much smaller tranches than conventional bonds, making them less marketable. Also, their overall expected return is likely to be lower than on conventional bonds (has an inflation risk premium in addition to expected inflation in the expected return formula, whereas ILB's provide only inflation protection - no upside above that) - this reduces demand and the ease at which they can be sold - lower marketability
Other things might include the availability and size of issues of particular terms to match investors' liabilities - if only limited ones available, then reduced demand and marketability, and with small issues, dealing costs are likely to be higher.
Hope this makes sense! (This is in general - in particular cases when inflation uncertainty is extremely high, ILB's may be more marketable because people value the inflation protection more than the potential upside of the risk premia, but generally not the case)