By way of supplementing Mari-Lou A's answer, I note that Chicago Manual of Style is by no means alone in asserting that there should be no comma before an ampersand when one appears before the last item in a series. Here are the guidelines that various style manuals provide on this point.

From University of Chicago, Chicago Manual of Style, sixteenth edition (2010):

6.21 Omitting serial commas before ampersands. When an ampersand is used instead of the word and (as in company names), the serial comma is omitted.

Winken, Blinken & Nod is a purveyor of nightwear.

From Merriam-Webster, Webster's Standard American Style Manual (1985):

When an ampersand is used between the last two elements in a series, the comma is omitted.

the law firm of Shilliday, Fraser & French

From U.S. Government Printing Office, A Manual of Style (1986):

The comma is omitted— ... 8.57. Before ampersand (&). (For exception, see rule 15.29.)

Brown, Wilson & Co.

Mine, Mill & Smelter Workers

The exception given in rule 15.29 involves index entries in which a surname appears as part of a company name that also includes an ampersand; USGPS declares that, in such cases, and the company name should be indexed alphabetically by the surname, in the following form:

Brown, A.H., & Sons (not Brown & Sons, A.H.)

Bu this exception doesn't involve a series, so it isn't relevant to the posted question.

The only formal style that directly contradicts the generally approved rule not to use a comma before & in a series is APA (American Psychological Association) style. According to Hodges' Harbrace Handbook, revised thirteenth edition (1998):

Use the ampersand (&) to separate the authors names.

A work by more than two authors

One recent study has shown that people who fear failure are not susceptible to hypnosis (Manganello, Carlson, Zarillo, & Teeven, 1985)

I should perhaps also note that many style guides oppose the use of ampersands except in proper names. For example, The Associated Press Stylebook (2007) includes this guideline:

ampersand (&) Use the ampersand when it is part of a company's name or composition title: House & Garden [magazine title], Procter & Gamble, Wheeling & Lake Erie Railway.

The ampersand should not otherwise be used in place of and.

So AP style would require the OP's example sentence to be rendered as follows:

The same products attracted all European countries to India: spices, silk, and cotton.

Although the Oxford Guide to Style (2002) generally agrees with AP on this point, it adds an interesting exception:

Avoid ampersands except in names of firms that use them, established combinations (e.g., R & D, R & B, C & W),and in some lexicographic work. Occasionally they may be convenient for clarification: in cinnamon & raisin and onion bagels are available the ampersand makes clear there are two rather than three types on offer.

The Oxford Dictionary for Writers and Editors (2000), follows up on this point by more explicitly asserting the ampersand's ability to indicate a nested pairing:

ampersand = &, used in some formulae, references, and lexicographic work, in Acts of Parliament, and in business names. Since it may imply a closer relationship than and, the ampersand can be useful in grouping items: 'cinnamon & raisin and onion' are two, not three, types of bagels." ...

This notion of using an ampersand to nest a compound entry within a longer series appears to be the central point of Ant_222's answer, as well. Still, Oxford's discussion of bagel types does not constitute not an endorsement of using an ampersand in place of and in a series such as "spices, silk, and cotton."