Because lookaheads are "zero length" matches. That is they do not consume any of the string that they match. So, in your example,

1welcometo india.
^...............is matched and consumed by ([0-9]+)
 ^^^^^^^........is matched but not consumed by (?=welcome)
 ^..............so "to india" tries to match here and fails.
[download]

This does match though:

$str = '1welcometo india';;

$str =~ m/([0-9]+)(?=welcome)welcometo india/ and print 'matched';;
matched
[download]

It's called 'lookahead' because it looks ahead but doesn't travel there.