First and foremost is quantity and variety. It is estimated that there are 10,000,000 active websites today, and thousands being added every day. With food, generally a family eats large quantities of the same diet, whereas business or educational use of the internet requires the use of search engines which can expose the user to tens of new websites with each new search.
Second, the halachic issues involved with food are very different than internet content. In Jewish law, kosher and non-kosher food is very clearly defined, this is not the case with webpages. Jewish law being a legal system that encompasses both criminal and civil law, generally limits itself to actions and rarely address thoughts, ideas or entertainment. It is much harder to determine exactly which content is forbidden, discouraged or simply to be avoided. Furthermore, while it is clear that internet has caused problems in religious families, to some degree it has also provided benefits. This has resulted in a legal ruling which discourages internet in almost all cases, but has a few significant exceptions (parnasa, research by professionals, jewish outreach). Kosher food has no equivalent set of rulings.
Thirdly, there is a psychological and addictive factor which is not normally present with food products. Rabbinical authorities reviewed evidence that individuals who would not otherwise engage in objectionable activities, do so because of the attractive and anonymous nature of the internet. As one example, the cases of several families were heard where the husband left the family due to a relationship developed through chat forums - something that had no counterpart in similar hareidi families who made no use of the internet.