I work as a TA in Primary School and from the perspective I see it, there are not necessairily more children with SEN, but there are more children that struggle to meet the expectations set by schools and the gov. The school curriculum has changed dramatically over the 30 years or so that I've worked in schools.
When I first started teaching in Y1 the curriculum was far less formal and demanding than it is now. There was plenty of time for free play- my classroom had sand, water, a craft area, a role play area, the children worked in small groups with an adult in turn while others played and then they rotated. There was time in the day for story time, an hour outside at lunch and two breaks a day. Year one maths involved practising counting to 20, adding, subtracting making patterns, comparing sizes, shapes, etc. with numbers generally within 20 or more for the more able. English involved writing simple sentences, basic stories, reading simple books and phonics.
Now in Y1 there is no room for free play, lessons are whole class based with children expected to watch PowerPoints at the start of each one, there is little differentiation, there is no sand, water or role play area. Any play is directed and has a given outcome or task set. In maths children are expected to to understand partitioning, tens and units, multiplication, division, fractions, read, write and recognise numbers to 100, know odd and even numbers.
Whilst all this might not be difficult for some children- for many 5 year olds it is and if they can't achieve the expectations, sit still for extended periods etc. they end up on the SEN register.
Therefore, whilst I think there are more children coming into school with lower levels of literacy than previously, perhaps because of changes in society and parenting- there are also expectations and pressures within schools that make some perfectly normal children appear to struggle.